Refactor Composer rendering for smoother animations

Also fixes a couple of miscellaneous bugs:
- Minimise the Composer when clicking the preview button in full-screen mode on desktop.
- Minimise the Composer when clicking the link to the discussion/post in the header on mobile/full-screen mode.

less/forum/Composer.less

@@ -39,12 +39,15 @@
   h3 {
     margin: 0;
     line-height: 1.5em;
+    color: @secondary-color;
 
     &, input, a {
-      color: @secondary-color;
       font-size: 14px;
       font-weight: normal;
     }
+    input, a {
+      color: inherit;
+    }
     input {
       font-size: 16px;
       width: 500px;
@@ -185,6 +188,10 @@
         white-space: nowrap;
         overflow: hidden;
         text-overflow: ellipsis;
+
+        h3 {
+          color: @header-control-color;
+        }
       }
     }
     h3 {