After installing a theme, display a link to "install another theme"

This issue is the obvious followup to #1446650: After installing module display link to "install another module" → .

Problem/Motivation

Make installing another theme easier by linking to the install page after a new theme is installed.

Proposed resolution

Add a "Install another theme" link after a new theme is installed.

Also, to match setup of core/lib/Drupal/Core/Updater/Module.php, set up row of links in the following order:

• Install another theme
• Enable newly added theme
• Administration pages

Remaining tasks

Reviewed and Tested by the Community.

From core/lib/Drupal/Core/Updater/Module.php:

 /**
   * {@inheritdoc}
   */
  public function postInstallTasks() {
    // Since this is being called outsite of the primary front controller,
    // the base_url needs to be set explicitly to ensure that links are
    // relative to the site root.
    // @todo Simplify with https://www.drupal.org/node/2548095
    $default_options = [
      '#type' => 'link',
      '#options' => [
        'absolute' => TRUE,
        'base_url' => $GLOBALS['base_url'],
      ],
    ];
    return [
      $default_options + [
        '#url' => Url::fromRoute('update.module_install'),
        '#title' => t('Install another module'),
      ],
      $default_options + [
        '#url' => Url::fromRoute('system.modules_list'),
        '#title' => t('Enable newly added modules'),
      ],
      $default_options + [
        '#url' => Url::fromRoute('system.admin'),
        '#title' => t('Administration pages'),
      ],
    ];
  }

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User interface changes

Before:

After:

API changes

None