Add relationships to client models

No issue
- Removed tabs from tag.js (why didn't jshint catch this?)
- Removed superfluous `activate` in SettingsIndexRoute
- updated `UserModel` and `TagModel` to have `created_by, updated_by` be references to `user` objects.
- updated `UserModel` to use `moment-date` instead of `date`
Merge pull request #3345 from novaugust/model-relationships

Add relationships to client models