Add webpack for frontend needs

[mimischi]

I'm getting tired of managing frontend libraries by hand. webpack looks promising to solve this issue.

Adding django-webpack-loader and with the help of this blog post (minus the React part), the project now ships almost without any external JavaScript libraries. The only exception is tagEditor which does not have any npm package.

Changelog

• Use yarn to pull in all frontend dependencies.
• Use webpack to compile a main.js bundle including JavaScript code and CSS (minified in production).
  • The user only makes one HTTP request to the server and not several to retrieve the frontend libraries.
  • It's more convinient to add new JavaScript dependencies, as long as we are using this frontend
• Get rid of the unmaintained django-bootstrap-datetimepicker3 package. Replace with normally embedded bootstrap-datetimepicker3.
• Change the DateTimePickers to be inline always, thus hiding the input field from the user.
• Update formatting and translations.
• Get rid of from __future__ imports. The codebase should be Python 3 only now.

[codecov-io]

Codecov Report

Merging #444 into develop will increase coverage by 0.41%.
The diff coverage is 73.52%.

@@             Coverage Diff             @@
##           develop     #444      +/-   ##
===========================================
+ Coverage    85.39%   85.81%   +0.41%     
===========================================
  Files           50       50              
  Lines         1623     1621       -2     
===========================================
+ Hits          1386     1391       +5     
+ Misses         237      230       -7

Impacted Files	Coverage Δ
clock/shifts/utils.py	75.34% <ø> (ø)	⬆️
clock/profiles/middleware.py	94.11% <ø> (ø)	⬆️
clock/pages/mixins.py	87.5% <ø> (ø)	⬆️
clock/contracts/views.py	88.09% <ø> (ø)	⬆️
clock/pages/utils.py	95.45% <ø> (ø)	⬆️
clock/contact/urls.py	100% <ø> (ø)	⬆️
clock/contracts/fields.py	40.74% <0%> (ø)	⬆️
clock/profiles/views.py	56.89% <100%> (ø)	⬆️
clock/contracts/urls.py	100% <100%> (ø)	⬆️
clock/pages/views.py	100% <100%> (ø)	⬆️
... and 13 more

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