Async javascript load should only move script tags with type="text/javascript"

Currently the fixes that were done for HSTTWO-2845 will break code that uses <script> tags for anything else than execution javascript, e.g. Handlebars templates.

We should improve the code that moves script elements and restrict it to elements with type="text/javascript" only. Note that in HTML5 the default value for the type attribute is "text/javascript" (if it is missing).

Examples of Handlebars templates:

<script id="entry-template" type="text/x-handlebars-template">
Hello, <strong>firstName lastName</strong>!
</script>

<script type="text/x-handlebars">
Hello, <strong>firstName lastName</strong>!
</script>