Remove Nbconvert template loading magic

This specifies the template names in the exporter class, rather than getting them from the module names, which seemed a bit too magical. This continues #4724, which I made from my master branch by accident. The problems @damianavila noted there are fixed.

Also, the latex templates got renamed from `latex/latex_foo.tplx` to `latex/foo.tplx`.

At present, when specifying a template to use, you have to specify the full template name, e.g. `html_basic`, not just `basic`. We may want to think more about this.