Fix hashtag search

When we lowercase the search it will succesfully do a case insenstive
match. Now #Linux will match #linux and #LINUX whereas previously it
would only match the exact case.

lib/pleroma/web/mastodon_api/mastodon_api_controller.ex

@@ -508,6 +508,7 @@ def hashtag_timeline(%{assigns: %{user: user}} = conn, params) do
       |> Map.put("type", "Create")
       |> Map.put("local_only", local_only)
       |> Map.put("blocking_user", user)
+      |> Map.put("tag", String.downcase(params["tag"]))
 
     activities =
       ActivityPub.fetch_public_activities(params)