Improve appearance of text editor buttons

- Change preview icon to Font Awesome Regular instead of Solid
- Give the preview button a tooltip
- Change submit button icon to paper plane (shown on mobile)
- Tweak mobile header button touch target
- Scroll overflowing text editor controls horizontally

less/forum/Composer.less

@@ -343,11 +343,14 @@
 }
 .TextEditor-controls {
   margin: 0;
-  padding: 15px 0;
+  padding: 10px 0;
   list-style-type: none;
+  overflow-x: auto;
+  white-space: nowrap;
 
-  & li {
+  li {
     display: inline-block;
+    margin-right: 10px;
   }
 }
 
@@ -357,10 +360,6 @@
     padding: 10px 20px;
     border-top: 1px solid @control-bg;
 
-    li {
-      margin-right: 5px;
-    }
-
     .fullScreen & {
       margin: 0;
       border-top: 0;