Use an inline template with _.template to render the gallery's HTML

If we want to get that long string of HTML out of our JavaScript, the most direct way is to move it straight back into gallery.php—By enclosing the markup in a `<script type="text/html">` it will not be rendered by the browser, so we can write the markup out right in our document and then use [Underscore.js' `_.template()` method](http://underscorejs.org/#template) to convert it to a JavaScript template.