Return profile URL when available instead of actor URI for MastodonAPI mention URL

Fixes #1165

lib/pleroma/web/mastodon_api/views/account_view.ex

@@ -28,7 +28,7 @@ def render("mention.json", %{user: user}) do
       id: to_string(user.id),
       acct: user.nickname,
       username: username_from_nickname(user.nickname),
-      url: user.ap_id
+      url: User.profile_url(user) || user.ap_id
     }
   end