This change adds a repository form view for GitHub that modifies the

form when a GitHub App installation account is selected. When an app

installation is selected for the account, this restricts the available

plans to just the new app plan, and shows only the "Repository Name"

field.
This also labels accounts so app installation accounts and PAT-based

accounts that have the same username can be distinguished.

Used the repository form for GitHub repos that were on both

  app-install and PAT linked hosting service accounts. Verified that

  interaction worked as expected.
Ran unit tests