Add Hound to teams with admin access

* We discovered that `Octokit::Client#repo_teams` may return repo teams to which the current Hound user does not belong. When we attempt to add Hound to a team to which the current user does not belong, `Octokit::Client#add_team_member` returns false and `ActivationsController#create` returns a 502 status. This is causing some users to experience a "Repo failed to activate" error.
* In this branch:
  - Change logic in `GithubApi#add_user_to_org` so Hound is added to the first existing repo team   with admin access of which the current user is a member
  - Add user_teams.json fixture
  - Update tests
* https://trello.com/c/CX9NavUF