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/federation/federatingprotocol.go

@@ -125,6 +125,29 @@ func (f *federator) AuthenticatePostInbox(ctx context.Context, w http.ResponseWr
 		return ctx, false, fmt.Errorf("not authenticated: %s", err)
 	}
 
+	// authentication has passed, so add an instance entry for this instance if it hasn't been done already
+	i := &gtsmodel.Instance{}
+	if err := f.db.GetWhere([]db.Where{{Key: "domain", Value: publicKeyOwnerURI.Host, CaseInsensitive: true}}, i); err != nil {
+		if _, ok := err.(db.ErrNoEntries); !ok {
+			// there's been an actual error
+			return ctx, false, fmt.Errorf("error getting requesting account with public key id %s: %s", publicKeyOwnerURI.String(), err)
+		}
+
+		// we don't have an entry for this instance yet so dereference it
+		i, err = f.DereferenceRemoteInstance(username, &url.URL{
+			Scheme: publicKeyOwnerURI.Scheme,
+			Host:   publicKeyOwnerURI.Host,
+		})
+		if err != nil {
+			return nil, false, fmt.Errorf("could not dereference new remote instance %s during AuthenticatePostInbox: %s", publicKeyOwnerURI.Host, err)
+		}
+
+		// and put it in the db
+		if err := f.db.Put(i); err != nil {
+			return nil, false, fmt.Errorf("error inserting newly dereferenced instance %s: %s", publicKeyOwnerURI.Host, err)
+		}
+	}
+
 	requestingAccount := &gtsmodel.Account{}
 	if err := f.db.GetWhere([]db.Where{{Key: "uri", Value: publicKeyOwnerURI.String()}}, requestingAccount); err != nil {
 		// there's been a proper error so return it