Just remove all items and re-add them on index

- Less code, and keeps items on life support (i.e. not in the feed but
  kept around via keep_time or keep_unread ) toward the end instead of
the beginning so tag[0] is always the most recent top item.
Add feed.index test
Add experimental TT-RSS server compatibility plugin

- Read-only in this version, but WIP
- Auth is non-functional