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.


git-svn-id: https://develop.svn.wordpress.org/trunk@38672 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/file.php

@@ -1261,7 +1261,7 @@ foreach ( (array) $extra_fields as $field ) {
 ?>
 	<p class="request-filesystem-credentials-action-buttons">
 		<button class="button cancel-button" data-js-action="close" type="button"><?php _e( 'Cancel' ); ?></button>
-		<?php submit_button( __( 'Proceed' ), 'button', 'upgrade', false ); ?>
+		<?php submit_button( __( 'Proceed' ), '', 'upgrade', false ); ?>
 	</p>
 </div>
 </form>