Integrate feed-specific setup into general setup

The special page setup was previously handled separately for feed output
to restrict the request arguments to a small subset of possible filter
options. This behavior was removed in 52b59f0685.

This makes the main difference between the normal and the feed setup that
custom filter options were not loaded for feed output. Support for custom
filter options was introduced with fcbdd58326 but did not touch the feed
setup, preventing filters from being initialized when requesting a feed
(see bug 57201).

After adding support for custom filters to the feed setup, the only
remaining difference is a different default limit, and the support for
subpage syntax for non-feed output.

This commit merges the feed setup with the standard setup.

Furthermore, in `SpecialRecentchangeslinked`, the extra argument parsing
was made also redundant with 52b59f0685.

Bug: 57201
Change-Id: Ia8ba28efb96da9df5d7252278d46ff88a143368c