theme_options filter isn't always triggered for exhibit themes

Once I've saved an exhibit's theme configuration, I can no longer access the values via the theme_options filter. Omeka_Form_ThemeConfiguration::init() does call this filter via Theme::getOptions() if nothing has been saved, but Exhibit Builder never calls the filter if theme options have been loaded from the exhibits table.

Steps to reproduce

1. Implement the theme_options filter in a plugin class like so:

protected $_filters = ['theme_options'];

public function filterThemeOptions($serializedOptions, $args)
{
    echo 'asdf';
    return $serializedOptions;
}

2. Edit an exhibit, switch to another theme, and configure it.
3. Notice the 'asdf' at the bottom of the page.
4. Save the theme configuration form.
5. Press Configure again.

Expected behavior

'asdf' should still be at the bottom of the page.

Actual behavior

'asdf' is not present.

Start with Omeka_Form_ThemeConfiguration::init() and Theme::getOptions(), then trace how Exhibit Builder loads saved theme options from the exhibits table. Reproduce the configuration flow described in the issue and verify that theme_options is invoked both before and after saving configuration.