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-includes/general-template.php

@@ -481,7 +481,7 @@ function wp_login_form( $args = array() ) {
 			' . $login_form_middle . '
 			' . ( $args['remember'] ? '<p class="login-remember"><label><input name="rememberme" type="checkbox" id="' . esc_attr( $args['id_remember'] ) . '" value="forever"' . ( $args['value_remember'] ? ' checked="checked"' : '' ) . ' /> ' . esc_html( $args['label_remember'] ) . '</label></p>' : '' ) . '
 			<p class="login-submit">
-				<input type="submit" name="wp-submit" id="' . esc_attr( $args['id_submit'] ) . '" class="button-primary" value="' . esc_attr( $args['label_log_in'] ) . '" />
+				<input type="submit" name="wp-submit" id="' . esc_attr( $args['id_submit'] ) . '" class="button button-primary" value="' . esc_attr( $args['label_log_in'] ) . '" />
 				<input type="hidden" name="redirect_to" value="' . esc_url( $args['redirect'] ) . '" />
 			</p>
 			' . $login_form_bottom . '