Isolate the logic to validate a gitlab username so it can be reused

Problem/Motivation

Currently, the logic to validate a git_username is highly tied to the form validation hooks. This validation can be really useful in other places and should not be tied to "form_set_error" calls or similar.

Steps to reproduce

Try to validate a git_username outside that form context, you can't without duplicating validation rules.

Proposed resolution

Have a separate re-usable method that does the validation and then have the form use it.

Remaining tasks

Work on it.

User interface changes

None.

API changes

There will be a new re-usable function to validate gitlab usernames after this.

Data model changes

None.