Mako templates should be named .mako

[fghaas]

It's pretty confusing that the Mako templates we use are named .xml, when they're really not valid (nor even well-formed) XML. It trips up syntax highlighters (such as Emacs' web-mode), too. I suggest we expect Mako templates to be named <filename>.mako, such as course.xml.mako.

This will also make things clearer to users when, at any time, we support template engines other than Mako (say, Jinja2).