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

 * errorhandler: don't try to truncate null strings
 * UrlHelper::rewrite_relative: fix undefined offset warnings for URLs
that lack schema/host (data: etc)

Intentionally skipping the line 66 one for now; adding an 'is_array' check clears the warning, but there's a larger topic of how to handle an invalid '' that doesn't result in an array.

------ ---------------------------------------------------------------------
Line   classes/urlhelper.php
------ ---------------------------------------------------------------------
66     Offset 'path' on array{scheme: string} in isset() does not exist.
165    Parameter #2 $associative of function get_headers expects bool, int
        given.
167    Parameter #2 $associative of function get_headers expects bool, int
        given.
278    Negated boolean expression is always true.
309    Negated boolean expression is always true.
------ ---------------------------------------------------------------------

remove SELF_USER_AGENT custom constant, replaced with configurable Config::HTTP_USER_AGENT / Config::get_user_agent()

 - support invoking specifying owner URL element/attribute
 - restrict mailto/magnet/tel schemes for A href
 - allow some data: base64 image types for IMG src

Sanitizer::sanitize():

 - when checking href and src attributes, pass element tagname and attribute to rewrite_relative()

UrlHelper::rewrite_relative: use base URL path if relative url path is not absolute (experimental)

 * af_redditimgur: reparent generated iframes outside of post table

I tried applying to only $error and only $old_error, but both appear to be needed.

Log entries:
E_NOTICE (8) 	classes/urlhelper.php:464 	Trying to access array offset on value of type null
1. classes/urlhelper.php(464): ttrss_error_handler(8, Trying to access array offset on value of type null, classes/urlhelper.php, 464, [)
2. classes/rssutils.php(464): fetch([{"url":"https://some.url.rss","login":"","pass":"","timeout":15,"last_modified":"Sat, 31 Aug 2019 15:22:31 GMT"})
3. update.php(235): update_rss_feed(732, 1)

- fix IDN hostnames not being converted properly