fix: set button font-size for modals

Atlaskit at times will have localized styling for font-size and
sometimes will not. The button component will inherit its
font-size whereas selectors have localized font-size of 14px. For
consistency, the cancel/submit buttons on the atlaskit modals
will also have 14px. The atlaskit story book examples also use
buttons with 14px font-size.

css/_variables.scss

 $rateStarActivity: #165ecc;
 $rateStarSize: 34px;
 
+/**
+ * Modals
+ */
+$modalButtonFontSize: 14px;
+
 /**
  * Notifications
  */

css/modals/_dialog.scss

         border-bottom: 1px solid $auiBorderColor;
     }
 }
+
+.modal-dialog-footer {
+    font-size: $modalButtonFontSize;
+}

react/features/base/dialog/components/Dialog.web.js

      */
     _renderFooter() {
         return (
-            <footer>
+            <footer className = 'modal-dialog-footer'>
                 <AKButtonGroup>
                     { this._renderCancelButton() }
                     { this._renderOKButton() }