* Change `/api/v1/teams/[team_id]/members` from taking `id` to `user_id`.
* Not even the admin panel was using this endpoint so doesn't seem that drastic of a change
* Add UI to handle team member removal
* Format Javascript and CSS files with `prettier`: `prettier --write 'CTFd/themes/**/*'`
* Format Python with `black`: `black CTFd` & `black tests`
* Travis now uses xenial instead of trusty.
* Fixes `populate.py` to assign captains to teams.
* Adds `ondelete='CASCADE'` to most ForeignKeys in models
* Closes#794
* Test reset in team mode to test removing teams with captains
* Test deleting users/teams with awards to test cascading deletion
* `gen_team()` test helper now creates users for the team and assigns the first one as captain
* Added `Challenges.flags` relationship and moved the `Flags.challenge` relationship to a backref on `Challenges`
