Add frontend support for plugins

This is quite a huge task (and may be split into smaller chunks), but nevertheless important. GDAPS needs to support a (or more) frontend(s) like Vue.js, React, Angular etc. by supporting "split" up frontend code packaged with each GDAPS/Django app, preferrably using DRF.

This plugin needs either

• to be compiled by e.g. webpack into one static/ folder before Django start using something like a special command ('./manage.py compilevueor more generally./manage.py compile_gdaps_client` or the like),
• or to be delivered into Django templates, mixed with frontend directives (the delimitors in e.g. Vue, {{, }} then have to be changed, which is a bad idea if 3rd party libs should be used too), and compiled in a step before or after the Django template compilation.

The first method seems the preferred to me.