Fix the position of the Mastodon mascot in the UI

The Mastodon mascot was previously anchored to the bottom, and that was since broken. This restores that behaviour!

It also disables the double-scrollbar behaviour that was caused by this area allowing overflow-y in addition to its parent doing so.

app/assets/stylesheets/components.scss

@@ -1149,10 +1149,9 @@ a.status__content__spoiler-link {
 
 .getting-started {
   box-sizing: border-box;
-  overflow-y: auto;
   padding-bottom: 235px;
   background: image-url('mastodon-getting-started.png') no-repeat 0 100% local;
-  height: 100%;
+  flex: 1 0 auto;
 
   p {
     color: $color2;