Add translations config group.

Summary:
Adds the translations group as per T2255. Currently `translation.override` is
`wild` -- it should be changed to dict<string, string> when that exists.

Also fixes a small bug from D4326 which caused "class" types to not ever
validate.

Test Plan:
- Looked at the settings.
- Successfully saved a setting relating to classes.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2255

Differential Revision: https://secure.phabricator.com/D4350