Draft: WIP: Add search box to susimail

Searches subject and recipients only. Not a full-text search. No index is generated or persisted.
Search is per-folder only, from the folder (LIST) view state only.
Supports iteration and paging within results.
This is a POST search only. No js/XHR support.

Splits up the single HTML form in folder view to 5 forms, as necessary to
do this sanely, and also as prep for js search later.

WIP, may contain bugs and some small unrelated changes, not for 2.5.0.

TODO: Testing, cleanup, verify all state transitions handled, more planning for js, and then js.