Update Phabricator to work with more modular translations

Summary:
Ref T7152. Ref T1139. This updates Phabricator so third-party libraries can translate their own stuff. Also:

• Hide "All Caps" when not in development mode, since some users have found this a little confusing.
• With other changes, adds a "Raw Strings" mode (development mode only).
• Add an example silly translation to make sure the serious business flag works.
• Add a basic British English translation.
• Simplify handling of translation overrides.

Test Plan:

• Flipped serious business / development on and off and saw silly/development translations drop off.
• Switched to "All Caps" and saw all caps.
• Switched to Very English, Wow!
• Switched to British english and saw "colour".

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7152, T1139

Differential Revision: https://secure.phabricator.com/D11747