Enable translations based on Edition

Objective

GitLab JiHu has needs to have specific translations for language in GitLab and needs to override this in their own repository.

Proposal

Enable different translations based on JH edition. See gitlab-jh/gitlab#145 (comment 623873496) for more details.

• Generate .po files from jh/locale for JiHu edition
• Allow for locale selection in JH distribution to read from jh/locale instead of locale

User	Edition	Language
User A	EE	English (EE translation)
User B	JH	English (JH translation)
User C	JH	Simplified Chinese (JH translation)

Implementation

Details described at #337133 (comment 640563485)

1. Look up translations from jh/locale then fallback to locale
   • Backend Ruby !68573 (merged)
   • Frontend JavaScript
2. Generate keys in jh/locale that aren't present in EE and FOSS GitLab
3. Translate JiHu specific versions of the English string