Adding a TemplateRepoProvider model and a single subclass for GitHub integration.  TemplateRepos now belong to a TemplateRepoProvider with the GithubTemplateRepoProvider being the default.  The user's GitHub api key is migrated from the User model to the registered GithubTemplateRepoProvider instance's TemplateRepoProviderCredentials.  And the UserSerializer has been updated to ensure backwards compatibility with the panamax-ui use of the API.