[feature] Allow delivery to sharedInboxes where possible

[tsmethurst]

This PR adds logic for dereferencing, storing, and delivering to shared inboxes: https://www.w3.org/TR/activitypub/#shared-inbox-delivery

To do this, it adds a migration to add the field shared_inbox_uri to the gtsmodel.Account. This field is a string pointer, with three possible settings:

• null -- we don't know yet whether this account has a SharedInboxURI, we need to look it up
• a url string -- the discovered SharedInboxURI of the account
• an empty string -- we looked it up but the account doesn't have a shared inbox

Logic for looking up the shared inbox of an account has been added to the GetRemoteAccount function in the dereferencer. When that function is called, and we have a null value for SharedInboxURI, the account will be dereferenced (if it hasn't already) to discover the shared inbox address.

This means that shared inboxes will gradually be discovered by your instance as you use it, rather than a bunch of calls happening all at once after updating, potentially breaking things.

If admins want to preserve the old delivery behavior, they can set the new config option instance-deliver-to-shared-inboxes to false :) Then, shared inboxes will still be discovered, they just won't be used. This defaults to true.

[NyaaaWhatsUpDoc]

Have made a few comments, but otherwise looks good :)