This adds the beginning of the new "Connected Service" page to the admin

site, replacing the old "Hosting Service Accounts" list. This currently

queries the hosting service accounts, groups them by service, and

renders a list of entries which link to the model form. The rendering

itself is designed to allow individual services to override the display

to add additional functionality.
Next steps include:

- Make the "Connect a service" button do something.

- Link through to repository management, including listing out

  repositories connected to each HostingServiceAccount and a link to

  create new repositories with the hosting service/account prepopulated.

- Enhance GitHub to be able to use the GitHub App manifest flow.

- Add a filtering UI for when there are many entries visible.

- Merge integration configuration into this page.

Ran unit tests.
Created some HostingServiceAccount entries for various services and

  validated the result.