Add GET endpoints for remote subscription forms

There are two reasons for adding a GET endpoint:

0: Barely displaying the form does not change anything on the server.

1: It makes frontend development easier as they can now use a link,
instead of a form, to allow remote users to interact with local ones.

lib/pleroma/web/router.ex

@@ -337,6 +337,7 @@ defmodule Pleroma.Web.Router do
     pipe_through(:pleroma_html)
 
     post("/main/ostatus", UtilController, :remote_subscribe)
+    get("/main/ostatus", UtilController, :show_subscribe_form)
     get("/ostatus_subscribe", RemoteFollowController, :follow)
     post("/ostatus_subscribe", RemoteFollowController, :do_follow)
   end