Make favicon overlay optional

Problem/Motivation

It would be nice to be able to stop environment_indicator from putting an overlay on the favicon for certain environments, especially environments where users who can see the favicon are frequently logged in.

Proposed resolution

While there are internal options for turning it on and off, it's locked to "on":

      $js_settings = array(
        'environment_indicator' => array(
          'addFavicon' => TRUE,
          // Get the first letter out of the name.
          'faviconLabel' => $match['name'][0],
          'faviconColor' => $match['color'],
          'faviconTextColor' => $match['text_color'],
        ),
      );

It should be pretty easy to add a setting to turn it off.

Remaining tasks

1. Write a patch.
2. Review and RTBC.
3. Commit.

User interface changes

Adds an option to the settings form at admin/config/development/environment-indicator/settings which allows administrators to disable the favicon overlay.

API changes

None.