Don't show "Limited" or "Test" translations unless an install is in developer mode

Summary:
Ref T5267. Although translations with very few strings are already put into a "Limited Translations" group, this isn't necessarily clear and was empirically confusing to at least one user, who was surprised that selecting "Spanish" had no UI effect.

Instead, hide limited and test translations entirely unless the install is in developer mode.

Test Plan: In a non-developer-mode install, viewed translations menu. No longer saw translations with very few strings.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5267

Differential Revision: https://secure.phabricator.com/D16807