Add ability to use in sync Django applications

[joshuadavidthomas]

Since gidgethub is async-first, this library has started out with only the ability to use in an async context. However, I would wager the vast, vast majority of Django projects out there are not async. django-github-app should support both sync and async contexts for this reason.

There are already some stubbed out classes that I created to make a note of where the sync code will need to go:

• django_github_app.github.SyncGithubAPI
• django_github_app.views.SyncWebhookView

As well, the built-in events contained in django_github_app.events will need to be adjusted to provide sync handlers as well. I'm a little unsure about the best way to handle that..

All of the models already have sync and async methods, so there should need to be anything adjusted there to support sync.

TODO

• django_github_app.github.SyncGithubAPI (implement SyncGitHubAPI client #23)
• django_github_app.views.SyncWebhookView (implement SyncWebhookView and refactor GitHubRouter for sync support #31)
• Create sync versions of internal events in django_github_app.events (add sync handlers for internal webhook events #34)

[joshuadavidthomas]

I suppose for the events, the django_github_app.routing.GitHubRouter could be adjusted to route to sync or async based on.. something. Right now it is basically a loose wrapper around gidgethub.routing.Router so this needs to be more thought through, as that router expects async callbacks.

Maybe doing away with the wrapper and fleshing out the library's GitHubRouter to handle all the event routing itself is the way to go.

Still not sure how to route between sync and async for internal webhook events in a relatively seamless way for users of the library.