R1:ca6b9f66e198

Replace "search scope" with selectable default behavior

Summary:
Fixes T4365. See discussion in D8123.

This implements the most conservative solution of approaches discussed in D8123. Basically:

- When you search in primary search, we overwrite "query" in your default (topmost) search filter, and execute that.

This doesn't implement any of the other "sticky" stuff, where the query sticks around. Maybe we'll do that eventually, but it gets messy and could be confusing. Practically, this addresses the…