Administration: Improve the usage of the button CSS classes.

Introduces some consistency in the usage of the button CSS classes, fixes the
focus style for accessibility and responsiveness of the buttons.

- Adds the `button` class to all primary buttons make them responsive
- Removes all `secondary-button` classes and replaces it with button when needed. `button-secondary` shouldn't be used and exists just for backward compatibility reasons
- Replaces classes inside `submit_button()` with a shorthand for some buttons, and use an empty string for the default `button` class. Passing `button` is unnecessary
- Adjusts `get_submit_button()` to remove empty items

Props iseulde, dimchik, chris_d2d, mhowell, afercia.
Fixes #27314, #37138, #37448.

Built from https://develop.svn.wordpress.org/trunk@38672


git-svn-id: http://core.svn.wordpress.org/trunk@38615 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/customize-controls.css

@@ -1204,7 +1204,7 @@ body.cheatin p {
  * Widgets and Menus common styles
  */
 
-/* higher specificity than .wp-core-ui .button-secondary */
+/* higher specificity than .wp-core-ui .button */
 #customize-theme-controls .add-new-widget,
 #customize-theme-controls .add-new-menu-item {
 	cursor: pointer;