R1:f1c75a63823c

Allow construction of ApplicationSearch queries with GET

Summary:
Ref T3775 (discussion here). Ref T2625.

T3775 presents two problems:

# Existing tools which linked to `/differential/active/epriestley/` (that is, put a username in the URL) can't generate search links now.
# Humans can't edit the URL anymore, either.

I think (1) is an actual issue, and this fixes it. I think (2) is pretty fluff, and this doesn't really try to fix it, although it probably improves it.

The fix for (1) is…