The easiest way would be to do away with the content.php file for this template and move everything to your single-thumbnail-featured.php so you don’t mess up any other pages using content.php. Then I would simply move the_post_thumbnail() outside the col-md-8 into a full width col-md-12.
single-thumbnail-featured.php
//Loads the header in which there is a container and a row divs
<?php get_header(); ?>
<div class="col-md-12">
<?php if ( has_post_thumbnail() && ! post_password_required() ) : ?>
<figure class="entry-thumbnail">
<?php the_post_thumbnail( 'post-medium' ); ?>
</figure>
<?php endif; ?>
</div>
<div class="main-content col-md-8" role="main">
<?php while (have_posts()) : the_post(); ?>
<?php /* Copy everything EXCEPT the_post_thumbnail() stuff from content.php here */ ?>
<?php endwhile; ?>
</div>
// calls the sidebar which is inside a class col-md-4 div
<?php get_sidebar(); ?>