Add support for repository hosts

[akd-io]

This issue is not about integrating with these hosts, lettings users log in, and getting a hosted repo set up automatically.

This issue is about selecting the repo host in the form in order to let us know more about the final environment the repository is going to live in.

For example, #246 and #247 could make use of this information to determine what todos to show the user.

To do:

• [ ] Add support for GitHub
• [ ] Add support for GitLab
• [ ] Add support for BitBucket