Use a callback to create personal access tokens

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Close this issue

• Currently, the behavior is to use a generate class method to generate a token and assign it to the record.
• This could instead be done in a before_create callback, and the generate method becomes unnecessary.
• More context: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3749#note_12544395