Plugin/Theme Uploads: New capabilities; unify UIs; ensure compatibility with old filters.

Introduce upload_plugins and upload_themes capabilities to allow blocking of plugin and theme uploads, versus the old hacky (and not secure) ways of just hiding UI tabs. These are simply meta capabilities that map to install_plugins and install_themes.
 
Also:
 * Use the same nice design for the plugin upload screen as the theme upload screen.
 * Better compatibility for the old install_themes_tabs filter added in [29002]. see #28578.
 * Ensure using the install_plugins_tabs filter to remove the upload tab removes the new button.
 * Use 'Add Plugins' instead of 'Install Plugins' to match 'Add Themes'.

fixes #29236.


git-svn-id: https://develop.svn.wordpress.org/trunk@29634 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/plugin-install.php

@@ -195,7 +195,7 @@ function install_search_form( $type_selector = true ) {
  */
 function install_plugins_upload( $page = 1 ) {
 ?>
-	<h4><?php _e('Install a plugin in .zip format'); ?></h4>
+<div class="upload-plugin">
 	<p class="install-help"><?php _e('If you have a plugin in a .zip format, you may install it by uploading it here.'); ?></p>
 	<form method="post" enctype="multipart/form-data" class="wp-upload-form" action="<?php echo self_admin_url('update.php?action=upload-plugin'); ?>">
 		<?php wp_nonce_field( 'plugin-upload'); ?>
@@ -203,6 +203,7 @@ function install_plugins_upload( $page = 1 ) {
 		<input type="file" id="pluginzip" name="pluginzip" />
 		<?php submit_button( __( 'Install Now' ), 'button', 'install-plugin-submit', false ); ?>
 	</form>
+</div>
 <?php
 }
 add_action('install_plugins_upload', 'install_plugins_upload', 10, 1);