Use Django's JavaScript translation catalog

[courtneycb]

This PR enables the use of Django's JavaScript translation catalog, meaning we can now use gettext() to translate text in JavaScript files.

It also uses django-statici18n, a Django app that collects JavaScript catalogs into a single location. (Instead of generating it dynamically on each request).

The docs have also been updated.

The following changes were also made in the csu helper script:

• Compiles messages within the static command as compiling JavaScript translations creates a static file.
• Remove deletion of volumes as this deletes a Docker volume creating for storing node modules. Tidying Docker should be a user choice/action, not a project action.

Resolves #1409