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.
Created a new paste and manually selected the language
Edited a paste and changed the language.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: Korvin, epriestley, yelirekim
Maniphest Tasks: T11532
Differential Revision: https://secure.phabricator.com/D16463
