R1:9422596ebf2f

Converted Paste language selection to a typeahead

Summary: Fixes T11532. The language selection for pastes is now a typeahead that is backed by `pygments.dropdown-choices`. There is still a bit of weirdness around making "auto-detection" the default state. To actually select a different language, you first need to remove the "auto detect" option that is pre-populated in a new paste. Other than that, it works as intended.

Test Plan:
Create a new paste with a file extension that can be auto-detected.