Turn deletions into soft deletions (set the is_deleted flag) instead of
hard deletions (remove from database). The is_deleted flag actually
already existed, but it was never used before.
* Fix inconsistencies in how wiki pages were linked.
* Link directly to the wiki instead of to a title search that is expected
to redirect to the wiki.
/dmails is restricted to viewing dmails for CurrentUser only (due to
Dmail.visible in the index action). Remove owner_id from subnavbar links
in /dmails, and don't support it in /dmails?search[owner_id], since it
doesn't actually do anything.
Also removes related dead methods and fixes tests that didn't test owner_id properly.
