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/javascript/styles/mastodon/accounts.scss

@@ -464,6 +464,7 @@
   background: $simple-background-color;
 
   &__header {
+    background: $base-shadow-color;
     background-size: cover;
     background-position: center center;
     height: 90px;