* allow plugins to inject JS code into login form

If I enabled some user plugins via Preferences -> Plugins and then
disabled all of them, an error occurred:

Jul 16 22:34:20 php[100]: PHP Fatal error:  Uncaught TypeError: array_filter(): Argument #1 ($array) must be of type array, null given in /usr/share/webapps/tt-rss/classes/pref/prefs.php:1027
Jul 16 22:34:20 php[100]: Stack trace:
Jul 16 22:34:20 php[100]: #0 /usr/share/webapps/tt-rss/classes/pref/prefs.php(1027): array_filter()
Jul 16 22:34:20 php[100]: #1 /usr/share/webapps/tt-rss/backend.php(136): Pref_Prefs->setplugins()
Jul 16 22:34:20 php[100]: #2 {main}
Jul 16 22:34:20 php[100]:   thrown in /usr/share/webapps/tt-rss/classes/pref/prefs.php on line 1027

Apparently the issue was elevated from a warning to an error in PHP 8.0
[1].

[1] https://php.watch/versions/8.0/internal-function-exceptions

Add the missing space between "AND" and "MATCH" in MySQL search queries

 * expose said methods via CLI (update.php)
 * fix several invocations of deprecated functions
 * set stricter type hints on several method arguments

This reverts commit b91ffae2.

This reverts commit 9e557501.

prefs: move external filter tree onload method (which checks for inactive feeds, etc.) to FilterTree class

when determining feed-specific favicon, instead of using first match or generic fallback, go through entire list of determined favicon URLs

CURLAUTH_ANY as a fallback in case we got a 403.

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