use correct FeedOptions methods. fixes #4203

This makes feed.php use the correct methods for setting the feed type
and content-type header. It also adds the missing type definition for
RSS 1.0

�� Rector and PHPCS fixes

style fixes

feed: set tagline as subtitle. fixes #2239

Feed creation refactoring

This breaks up the humongous functions from feed.php into multiple
classes. To keep compatibility with existing Plugin events, the basic
principle of how the feed is assemb

Feed creation refactoring

This breaks up the humongous functions from feed.php into multiple
classes. To keep compatibility with existing Plugin events, the basic
principle of how the feed is assembled has not been changed:

* depending on the given mode and other options lose arrays of items are
gathered
* these items are then converted (again based on the various options)
into proper FeedItems
* the FeedItems are then added to the Feed

The conversion from loosely typed item data to something more structured
is now done by the FeedItemProcessor classes. Some very basic tests have
been added. It does not cover erverything but covers more than before
(which was nothing).

Manual testing before merging this is highly recommended. I am not
confident that I ported over everything correctly.

No new features have been added, but especially media support could and
should be improved in the future.

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