R1:3a2c2ae3c3ae

Implement a scope selector for the global search

Summary:
See M1433. Fixes T7266. Fixes T4475. Ref T7314.

Future work/notes/etc:

- Write the User Guide (see TODO).
- This might needs some design tweaks -- I think it's functionally almost-equivalent to the mock, but the UI isn't quite the same.
- (Mobile design is a touch off-looking I think?)
- When you use a custom query, the duplicate "magnifying glass" icons are a little weird. Maybe change one or the other.
- Maybe worth adding an "Open…