feeds: unify naming

classes/api.php

classes/counters.php

classes/feeds.php

classes/handler/public.php

classes/opml.php

classes/pref/feeds.php

classes/pref/filters.php

classes/rpc.php

classes/rssutils.php

include/functions.php

js/Feeds.js

plugins/af_comics/filters/af_comics_gocomics.php

plugins/af_comics/filters/af_comics_gocomics_farside.php

plugins/af_psql_trgm/init.php

plugins/af_readability/init.php

plugins/vf_shared/init.php