OAuthPlug: use user cache instead of joining

As this plug is called on every request, this should reduce load on the
database by not requiring to select on the users table every single
time, and to instead use the by-ID user cache whenever possible.

lib/pleroma/web/plugs/o_auth_plug.ex

@@ -47,15 +47,17 @@ def call(conn, _) do
   #
   @spec fetch_user_and_token(String.t()) :: {:ok, User.t(), Token.t()} | nil
   defp fetch_user_and_token(token) do
-    query =
+    token_query =
       from(t in Token,
-        where: t.token == ^token,
-        join: user in assoc(t, :user),
-        preload: [user: user]
+        where: t.token == ^token
       )
 
-    with %Token{user: user} = token_record <- Repo.one(query) do
+    with %Token{user_id: user_id} = token_record <- Repo.one(token_query),
+         false <- is_nil(user_id),
+         %User{} = user <- User.get_cached_by_id(user_id) do
       {:ok, user, token_record}
+    else
+      _ -> nil
     end
   end