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/plugin-install.php

@@ -265,7 +265,7 @@ function install_search_form( $deprecated = true ) {
 		<label><span class="screen-reader-text"><?php _e( 'Search Plugins' ); ?></span>
 			<input type="search" name="s" value="<?php echo esc_attr( $term ) ?>" class="wp-filter-search" placeholder="<?php esc_attr_e( 'Search Plugins' ); ?>" />
 		</label>
-		<?php submit_button( __( 'Search Plugins' ), 'button hide-if-js', false, false, array( 'id' => 'search-submit' ) ); ?>
+		<?php submit_button( __( 'Search Plugins' ), 'hide-if-js', false, false, array( 'id' => 'search-submit' ) ); ?>
 	</form><?php
 }
 
@@ -281,7 +281,7 @@ function install_plugins_upload() {
 		<?php wp_nonce_field( 'plugin-upload' ); ?>
 		<label class="screen-reader-text" for="pluginzip"><?php _e( 'Plugin zip file' ); ?></label>
 		<input type="file" id="pluginzip" name="pluginzip" />
-		<?php submit_button( __( 'Install Now' ), 'button', 'install-plugin-submit', false ); ?>
+		<?php submit_button( __( 'Install Now' ), '', 'install-plugin-submit', false ); ?>
 	</form>
 </div>
 <?php