User restrictions only on registration

Problem/Motivation

Add options to check user restrictions only when registering, not when logging in.

This would be helpful for cases where you want an 'Official' account on the site but don't want users creating usernames with the word 'Official' in them.

Steps to reproduce

- Create 'sage_official' account
- Add 'official' as a blocked word on user accounts
- Logging in as sage_official is no longer allowed

Proposed resolution

- Create option to block on login and/or registration.
- Allow existing accounts to log in

Note: whitelisting is an option, but in my case there are over 100 accounts that need to be whitelisted. It would be easier if it only blocks users from creating new accounts with the blacklisted text.

Remaining tasks

User interface changes

API changes

Data model changes