Start building whl and pex files on the CI server

[aronasorman]

Summary

• We now have additional make commands for creating a docker environment and building the whl files in there.
• We have a new directory: .buildkite, which contains scripts and steps to build the whl and pex files on https://buildkite.com/learningequality/kolibri (you need to get invited for now to see the output)

[codecov-io]

Codecov Report

Merging #897 into release will decrease coverage by -1.82%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           release   #897      +/-   ##
=========================================
- Coverage    71.82%    70%   -1.82%     
=========================================
  Files          144    139       -5     
  Lines         5395   5078     -317     
  Branches       672    599      -73     
=========================================
- Hits          3875   3555     -320     
- Misses        1456   1459       +3     
  Partials        64     64

Impacted Files	Coverage Δ
karma_config/vueLocal.js	91.66% <100%> (+0.75%)	✅
kolibri/utils/conf.py	56% <ø> (-3.38%)	❌
kolibri/auth/models.py	95.08% <ø> (-2.82%)	❌
kolibri/content/models.py	89.47% <ø> (-1.79%)	❌
kolibri/auth/api.py	94.39% <ø> (-1.2%)	❌
kolibri/auth/api_urls.py	100% <ø> (ø)	✅
kolibri/plugins/user/templatetags/user_tags.py
kolibri/plugins/user/kolibri_plugin.py
kolibri/plugins/user/views.py
kolibri/plugins/user/urls.py
... and 1 more

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[jamalex]

OK!