Replace make-templates.sh with Makefile

Beginnings of a Makefile which provides the following:

```
make app/js/templates.js
make all
make clean

make assert_templates_js_up_to_date
```

Note that I preserved the principle of generating a temporary file with
handlebars then moving it into place (which I presume was for
atomicity). However the tempfile is now generated by `mktemp` which is
safer than using the PID to derive a name.

Starts #227