changing default docker hub registry create to find_or_create_by
Adding the TemplateRepoProvider model and a single subclass for GitHub integration.
relating template repos to template repo providers
setting default GitHub public repo provider for new TemplateRepo (aka sources)
Constantizing the default template repo provider name
Allowing for templates to be saved to a repo within a specified template repo provider or the user's registered GitHub repo
Handling user updates with template repo provider credentials
adding tests and refactoring