Move re-requesting logic before parsing response body, otherwise it puts HTTP headers into XML body

 * add some basic tests for UrlHelper::rewrite_relative()
 * fix UrlHelper::rewrite_relative() to work better on non-absolute
   relative URL paths

Cherry-picked from https://git-gitea.tt-rss.org/fox/tt-rss/pulls/56,
while excluding changes in vendor/ (causes compatiblity issues with
PHP<8 [1]) and strftime-related ones (already re-applied in
https://git-gitea.tt-rss.org/fox/tt-rss/pulls/66).

[1] https://community.tt-rss.org/t/support-for-php-8-1/5089/9

getFeedUnread()

 * mark several symbols as @deprecated properly
 * replace uses of (deprecated) getFeedUnread() with Feeds::_get_counters()

rewrite_relative: prevent php warning when checking for unset content type in EXTRA_SCHEMES_BY_CONTENT_TYPE

rewrite_relative: whitelist specific schemes for URLs with 'known' content-types i.e. specified for enclosures

If for whatever reason $pdo holds a DDL lock on ttrss_entries, it could
block ORM's save, leading to a deadlock. To work around this, call
$pdo->commit() before ORM::for_table()->save().

generate a warning if plugin-generated content of HOOK_ARTICLE_BUTTON or _LEFT_BUTTON can't be parsed as valid XML

rssutils: rewrite several invocations of (deprecated) rewrite_relative_url() to UrlHelper::rewrite_relative()

 * add filter action to ignore feed-provided tags
 * simplify handling of various filter-provided tags
 * bump schema to 146

 imitate other controls like DropDownButton, etc.
 * rework several main toolbar items to use fox.form.Select instead of
other controls
 * replace HOOK_HEADLINE_TOOLBAR_SELECT_MENU_ITEM with
HOOK_HEADLINE_TOOLBAR_SELECT_MENU_ITEM2 because of markup change (option
instead of menuitem)
 * PluginHost: add some explicit typecasts to make intellephense shut up

 * auth_remote: add config option AUTH_REMOTE_POST_LOGOUT_URL

This reverts commit 14027ae0.

This reverts commit 72e21f89.

 * API: allow retrieving headlines for tag-based feeds (bump api level to 18)