feat(I18n): allow clients to override translations

See #1471 (closed). This ticket was previously implemented in a way that was hardcoded to affect all clients, which isn't what we wanted.

This MR rolls that back and adds a new back-end file which clients can use to override translations (including adding entirely new languages if they wish). That file, translationOverrides.js, resides in the config folder, which is a docker volume.

Note that this mechanism is not group-specific; it will modify translations across all groups running on the instance.

I also took the opportunity to clean up some unused translation entries, improve style, and fix translation bug #1505 (closed) which resulted in mislabeling of review reports.