Handle default kolibri behaviour from 0.15.x onwards to no longer collectstatic files

[rtibbles]

The pull request: learningequality/kolibri#7920 completed the reliance of Kolibri on using Whitenoise to serve precompressed static files, from the source directories.

This means that Kolibri no longer runs collectstatic as an ordinary part of its life cycle.

For nginx configurations to serve static files efficiently there are two options:

1. Transparently proxy the kolibri server, and rely on the cache headers set by Kolibri (Whitenoise sets intelligent cache headers by default). This option has the advantage that we can (alongside the work in Prepare kolibri server to work with the changes to support hashi in 0.15.x #82) focus on transparently proxying Kolibri endpoints, rather than more complex nginx configurations.

2. Configure the kolibri-server package to run collectstatic and carry on as before.

[jredrejo]

Thanks @rtibbles
I have already a proof of concept that works, just need to clean it up to create the pr for kolibri-server
With the current roadmap for kolibri 0.15 it does not seem to be urgent to do it, but if it we plan to release a beta version soon I'd do it in the next days.
@jonboiser are there any plans to do a kolibri 0.15 beta release in the short term?