Add the ability to view templates in a browser in development mode

This adds a simple mechanism to view any template file directly in a browser. The intention is that we can use this to build up reference HTML files which illustrate common UX patterns. We can then view the resulting pages without having to have a working implementation.

In development mode, the URL is /template followed by the path to the template file. For example, you can view the 404 page at:

    /template/404.html

You can also supply string parameters to the template by specifying them as query parameters in the URL.

e.g.  /template/my_template?name=Foo
Merge pull request #2524 from edx/andya/dev-show-template

Add the ability to view templates in a browser in development mode