Filter private calendar objects in shared calendars

The current logic for mod_rewrite relies on the fact that people have properly configured ownCloud, basically it reads from the `overwrite.cli.ur
l` entry and then derives the `RewriteBase` from it.

This usually works. However, since the ownCloud packages seem to install themselves at `/owncloud` (because subfolders are cool or so…) _a lot_ of people have just created a new Virtual Host for it or have simply symlinked the path etc.

This means that `overwrite.cli.url` is wrong, which fails hard if it is used as RewriteBase since Apache does not know where it should serve files from. In the end the ownCloud instance will not be accessible anymore and users will be frustrated. Also some shared hosters like 1&1 (because using shared hosters is so awesome… ;-)) have somewhat dubious Apache configurations or use versions of mod_rewrite from the mediveal age. (because updating is money or so…)

Anyhow. This makes this explicitly an opt-in configuration flag. If `htaccess.RewriteBase` is set then it will configure index.php-less URLs, if
admins set that after installation and don't want to wait until the next ownCloud version they can run `occ maintenance:update:htaccess`.

For ownCloud 9.0 we also have to add a repair step to make sure that instances that already have a RewriteBase configured continue to use it by copying it into the config file. That way all existing URLs stay valid. That one is not in this PR since this is unneccessary in master.

Effectively this reduces another risk of breakage when updating from ownCloud 8 to ownCloud 9.

Fixes https://github.com/owncloud/core/issues/24525, https://github.com/owncloud/core/issues/24426 and probably some more.

PATH_INFO will be empty at this point and thus the logic in base.php did not catch this. Changing this to "getRawPathInfo" will ensure that the path info is properly read.

Fixes https://github.com/owncloud/core/issues/23199

When using an background image in themes of type JPG, the current setting of owncloud's htaccess file does not allow to deliver these kinds of images as static content. Adding the file extensions as done in this commit, it works flawlessly.

In case Apache is configured with an `Alias` such as with the ownCloud packages the rewrite rules will fail when no valid RewriteBase is configured.

Otherwise `localhost/ocs-provider/` cannot be accessed if mod_rewrite is install
ed. Only affects master.

Two small security hardenings for our IE users and those with Adobe products. Aligns it more with https://github.com/twitter/secureheaders#secureheaders---

 Seems like Apache is inconsistent fun between versions. Let's remove it thus for now.

Some httpd versions have problem with the old logic leading to resourced served with multiple headers.

This can now be achieved using the new code signing.

When `DirectorySlash off` is set then Apache will not lookup folders anymore. This is required for example when we use the rewrite directives on an existing path such as  `/core/search`. By default Apache would load `/core/search/` instead `/core/search` so the redirect would fail here.

This leads however to the problem that URLs such as `localhost/owncloud` would not load anymore while `localhost/owncloud/` would. This has caused problems such as https://github.com/owncloud/core/pull/21015

With this change we add the `DirectorySlash off` directive only when the `.htaccess` is writable to the dynamic part of it. This would also make `localhost/owncloud` work again as it would trigger the 404 directive which triggers the redirect in base.php.

Makes `/core/img/favicon.ico` accessible again via web.

Fixes https://github.com/owncloud/core/issues/16164

See https://letsencrypt.readthedocs.org/en/latest/using.html#webroot

Required for routes that might otherwise collide with existing folders on the system

mod_rewrite as used by the front controller may require a `RewriteBase` in case the installation is done using an alias. Since we cannot enforce a writable `.htaccess` file this will move the `front_controller_active` environment variable into the main .htaccess file. If administrators decide to have this one not writable they can still enable this feature by setting the `front_controller_active` environment variable within the Apache config.

This changeset allows ownCloud to run with pretty URLs, they will be used if mod_rewrite and mod_env are available. This means basically that the `index.php` in the URL is not shown to the user anymore.

Also the not deprecated functions to generate URLs have been modified to support this behaviour, old functions such as `filePath` will still behave as before for compatibility reasons.

Examples:
http://localhost/owncloud/index.php/s/AIDyKbxiRZWAAjP => http://localhost/owncloud/s/AIDyKbxiRZWAAjP
http://localhost/owncloud/index.php/apps/files/ => http://localhost/owncloud/apps/files/

Due to the way our CSS and JS is structured the .htaccess uses some hacks for the final result but could be worse... And I was just annoyed by all that users crying for the removal of `index.php` ;-)

This reverts commit 68321efd.

This reverts commit d831c255.