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/views/stream_entries/_detailed_status.html.haml

@@ -8,7 +8,7 @@
       %span= acct(status.account)
 
   - if embedded_view?
-    = link_to "web+mastodon://follow?uri=#{status.account.local_username_and_domain}", class: 'button button-secondary logo-button', target: '_new' do
+    = link_to account_remote_follow_path(status.account), class: 'button button-secondary logo-button', target: '_new' do
       = render file: Rails.root.join('app', 'javascript', 'images', 'logo.svg')
       = t('accounts.follow')