Remote instance dereferencing (#70)

Remote instances are now dereferenced when they post to an inbox on a GtS instance.

    Dereferencing will be done first by checking the /api/v1/instance endpoint of an instance.
    If that doesn't work, /.well-known/nodeinfo will be checked.
    If that doesn't work, only a minimal representation of the instance will be stored.

A new field was added to the Instance database model. To create it:

alter table instances add column contact_account_username text;

internal/gtsmodel/instance.go

@@ -28,6 +28,8 @@ type Instance struct {
 	Terms string
 	// Contact email address for this instance
 	ContactEmail string
+	// Username of the contact account for this instance
+	ContactAccountUsername string
 	// Contact account ID in the database for this instance
 	ContactAccountID string `pg:"type:CHAR(26)"`
 	// Reputation score of this instance