Ref T4788. Fixes T10703.

In the longer term I want to put this on top of ApplicationSearch, but that's somewhat complex and we're at a fairly good point to pause this feature for feedback.

Inch toward that instead: provide more appropriate filters and defaults without rebuilding the underlying engine. Specifically:

• No "assigned" for commits (barely makes sense).
• No "assigned" for mocks (does not make sense).
• Default to "open" for parent tasks, subtasks, close as duplicate, and merge into.

Also, add a key to the search_document table to improve the performance of the "all open stuff of type X" query. "All Open Tasks" is about 100x faster on my machine with this key.

• Clicked all object relationships, saw more sensible filters and defaults.
• Saw "open" query about 100x faster locally (300ms to 3ms).