Created User interface for GL Account requests

Created User interface for gitlab account requests. GitlabAccountRequest model now has option one-to-one field for UserIdentifier (linked_user), so that users will be able to check the status of their GitlabAccountRequest, lowering moderator overhead--but users also have the ability to create GitlabAccount requests anonymously, if privacy is more important to them than being able to check on their request.

Created several two new view and url paths for creating account requests with and without user identifiers. Refined the PassUserIdentifierMixin to check if user_identifier in kwargs and pass if not, so that does not break when not. Adjusted success urls to route differently depending on if user_identifier was passed in kwargs.

Refactored username validation decorator by pulling out logic to check usernames against dictionary as a discrete function, so that it could be called outside of decorator (check_username).

Added {% if form.instance.linked_user %} logic to moderator template to display linked user_identifiers if present on Gitlab Account Requests.