Move help text html into a template file

The html for the help text is hardcoded currently. See shariff_help().

Although this is done in most modules this way currently I propose to add an extra template file for this that could look like this:

{#
/**
 * @file
 * Template for shariff help text html.
 *
 * Available variables:
 * - title: The help title.
 * - content: The main help text.
 *
 */
#}
<h3>{{ title }}</h3>
<p>{{ content }}</p>

The variables title and content can be provided in shariff_help() like this:

      $output = array(
        '#theme' => 'help_shariff',
        '#title' => t('About') . ' Shariff',
        '#content' =>  t('The Shariff social media buttons library by heise online enables website users to share their favorite content without compromising their privacy. For more information see <a href="@url">@url</a>.', array('@url' => 'https://github.com/heiseonline/shariff'))
      );
      return $output;

Of course the new template must be registered in shariff_theme():

'help_shariff' =>
      array(
        'variables' => array(
          'title' => NULL,
          'content' => NULL
        ),
        'template' => 'help--shariff',
      ),