Refactor MetagovRequestHandler

[shaunagm]

Moved this out of #175. Not thrilled with it as it stands, but at least I understand the code better?

This has not been tested yet.

Per conversation with @mashton, additional steps not addressed by this PR include:

• eventually moving "received authorized state" logic to http wrapper, since Django drivers don't need it
• handling client credentials grant type of auth2. currently we only do code grant, but for example Mastodon uses client credentials grants
• we need to delete communities if installation fails after the point in the process where the community is created
• we need to handle users revoking tokens/access

[gigxz]

Looks good to me

[shaunagm]

Okay, tested this on my dev server. I was able to install a Slack community, log in to that Slack community with my user, and set up a policy that responded to a message in Slack with another message.

@mashton, are there other things you think I should test, or should we go ahead and merge this?

[gigxz]

Testing Slack only tests the "platformwide handlers", so it wouldn't hit pass_to_plugin_instance. I'd try out Open Collective or Loomio too to test that part out!

[shaunagm]

Okay, I was able to verify that webhooks to OpenCollective are getting routed to pass_to_plugin_instance, although my dev server is then behaving weirdly. I think that's unrelated to this PR though, so I'm going to merge it.