Theme Boost: Simplify and harmonise flat_navigation.mustache

When working for a child theme, we stumble along the current existing structure in the flat_navigation.mustache file.
We think that this can be simplified.

Furthermore the structure of the items is not homogeneous because elements that have an action do not get div's and span's equal to elements which don't have an action.

Examples:

• Code of a topic section:

  <div class="m-l-0">

  <div class="media">

  <span class="media-left">

  <i class="icon fa fa-folder fa-fw " aria-hidden="true" aria-label=""></i>

  </span>

  <span class="media-body">section 1</span>

  </div>

  </div>

• Code of a section without an icon:

  <div class="m-l-1">

  My first course

  </div>

  Here, the media divs are missing.