Support AP-style mentions of centralized social media accounts

This allows users to mention users on the following non-ActivityPub
social media sites:

- twitter.com
- medium.com

It also adds missing error handling in federatePost().

activitypub.go

@@ -699,6 +699,10 @@ func federatePost(app *App, p *PublicPost, collID int64, isUpdate bool) error {
 			// I don't believe we'd ever have too many mentions in a single post that this
 			// could become a burden.
 			remoteUser, err := getRemoteUser(app, tag.HRef)
+			if err != nil {
+				log.Error("Unable to find remote user %s. Skipping: %v", tag.HRef, err)
+				continue
+			}
 			err = makeActivityPost(app.cfg.App.Host, actor, remoteUser.Inbox, activity)
 			if err != nil {
 				log.Error("Couldn't post! %v", err)

database.go

@@ -2652,6 +2652,17 @@ func handleFailedPostInsert(err error) error {
 func (db *datastore) GetProfilePageFromHandle(app *App, handle string) (string, error) {
 	handle = strings.TrimLeft(handle, "@")
 	actorIRI := ""
+	parts := strings.Split(handle, "@")
+	if len(parts) != 2 {
+		return "", fmt.Errorf("invalid handle format")
+	}
+	domain := parts[1]
+
+	// Check non-AP instances
+	if prefix, ok := fakeAPInstances[domain]; ok {
+		return "https://" + domain + "/" + prefix + parts[0], nil
+	}
+
 	remoteUser, err := getRemoteUserFromHandle(app, handle)
 	if err != nil {
 		// can't find using handle in the table but the table may already have this user without

silobridge.go

@@ -0,0 +1,10 @@
+package writefreely
+
+// fakeAPInstances contains a list of sites that we allow writers to mention
+// with the @handle@instance.tld syntax, plus the corresponding prefix to
+// insert between `https://instance.tld/` and `handle` (e.g.
+// https://medium.com/@handle)
+var fakeAPInstances = map[string]string{
+	"twitter.com": "",
+	"medium.com":  "@",
+}