Add discourse actor authorization

[angusmcleod]

@pmusaraj This is ready to go once #84 is merged.

• Authorizations now have a dedicated model and table (existing mastodon authorizations will be migrated)
• Discourse actor authorizations are now supported.
• The interface in user preferences has been improved.

Notes

To authorize a remote Discourse user, the ActivityPub auth redirect for the user's domain has to be in the site setting allowed user api auth redirects:

[url]/ap/auth/redirect/discourse

For example:

https://meta.discourse.org/ap/auth/redirect/discourse

[angusmcleod]

@pmusaraj This one is ready to go.

[angusmcleod]

@pmusaraj I've rebased and updated this so it's ready again. This should be the next one you review.

[pmusaraj]

Thanks @angusmcleod, will look into this today!