Make buildjs.sh script build a single minified JS file.

The file name includes a SHA hash for better cacheability.

The process is rather manual at the moment. The coffee.sh script
needs to be run manually after changing the source files,
the `JS_SHA` variable in `views.py` needs to be manually updated
every time the contents of the minified JS file change,
and the list of JS source files in `buildjs.sh` needs to be manually
updated if a new JS file is added or existing file removed.
The list of JS files is duplicated from https://github.com/edx-solutions/xblock-discussion/blob/e23a5537a9329ecdd28fda7127f392f2c75c0d47/discussion_app/views.py#L15 . We should have one canonical list.