Merge branch 'button-focus-style' into 'master'

Adds focus state style to markdown area buttons.

## What does this MR do?

It makes it possible to see when the button is focused when tabbing through the interface. This is for the new markdown buttons in the comment field.

See merge request !4867

app/assets/stylesheets/framework/markdown_area.scss

@@ -125,7 +125,8 @@
   border: 0;
   outline: 0;
 
-  &:hover {
+  &:hover,
+  &:focus {
     color: $gl-link-color;
   }
 }