Improve embeds (#7919)

* Make embeds cacheable by reverse proxy

* Make follow button on embeds open remote follow modal

Instead of web+mastodon://, also, turn the button blue, and add a
sign up prompt to the remote follow modal

app/controllers/statuses_controller.rb

@@ -46,7 +46,12 @@ class StatusesController < ApplicationController
   end
 
   def embed
+    raise ActiveRecord::RecordNotFound if @status.hidden?
+
+    skip_session!
+    expires_in 180, public: true
     response.headers['X-Frame-Options'] = 'ALLOWALL'
+
     render 'stream_entries/embed', layout: 'embedded'
   end