Add class to hide focus highlight

We use tabIndex to make elements selectable and therefore focused by screen readers. Doing this draws a blue border (in chrome at least) around the element - in some cases, we don't want this.

res/css/_common.scss

@@ -110,6 +110,10 @@ textarea {
     color: $primary-fg-color;
 }
 
+.mx_HiddenFocusable {
+    outline: none;
+}
+
 // .mx_textinput is a container for a text input
 // + some other controls like buttons, ...
 // it has the appearance of a text box so the controls