Break up DynamicWhiteNoise code

[dylanmccall]

Instead of generating everything in the initializer, we will use some more specific functions. This makes it easier to create a subclass of DynamicWhiteNoise, for example to support platforms that require special libraries for file access.

Summary

This patch was really helpful to support Android file access in endlessm/kolibri-installer-android#116. Previously, we had to clone all of kolibri_whitenoise.py into the Android app code, and with this change we're able to do something much more maintainable. There is still some evil involved to actually get the existing WSGI server to use our subclassed DynamicWhiteNoise, but it's much closer, and in general I think this can be classified as an innocent cleanup :)

Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
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• If this is an important user-facing change, PR or related issue has a 'changelog' label
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Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0a13.dev0_git.106.g164d269f.pex
Windows Installer (EXE)	kolibri-0.16.0a13.dev0+git.106.g164d269f-unsigned.exe
Debian Package	kolibri_0.16.0a13.dev0+git.106.g164d269f-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0a13.dev0+git.106.g164d269f-0.3.0.dmg
Source Tarball	kolibri-0.16.0a13.dev0+git.106.g164d269f.tar.gz
WHL file	kolibri-0.16.0a13.dev0+git.106.g164d269f-py2.py3-none-any.whl

[rtibbles]

I think I'd prefer to handle this a slightly different way that explicitly sets up the backend for handling different storage needs. As this doesn't seem to be pressing for anyone right now, I'm going to close this.