Add 'under_moderation' flag to user

- Change ProfanityFilter concern to ReviewFilter
- Move specs concerning ReviewFilter to separate spec
Add basic admin config for users

Currently only the under_moderation field is editable, and name and email is visible
make admins comments accept/reject same as questions
Merge pull request #174 from askaway/feature/moderated_users

Allow admins to flag a user as 'under moderation'.