Expose environment variables from database into build commands

[humitos]

Environment variables can be added from the Admin and they will be used when running build commands. All the variables for that particular project will be expose to all the commands.

This is the first approach for this feature. There are more decision that can be made around this and improvements. Although, this is the easiest way to implement this since all the environment variables are exposed to all the commands (by using existing code for this) and there are no checkings to be done when running each command.

I QA this under Docker and Local building and my custom env variable was exposed in both scenarios.

[humitos]

If this is the path that we want to follow, I'd add tests for this new code.

[agjohnson]

This doesn't close, #3992. It only wraps up the first phase.

[agjohnson]

Looking good! I think this is close, there were just some implementation details to firm up.

[codecov]

Codecov Report

Merging #4894 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4894      +/-   ##
==========================================
+ Coverage    76.6%   76.65%   +0.05%     
==========================================
  Files         158      158              
  Lines       10043    10057      +14     
  Branches     1268     1269       +1     
==========================================
+ Hits         7693     7709      +16     
+ Misses       2008     2007       -1     
+ Partials      342      341       -1

Impacted Files	Coverage Δ
readthedocs/doc_builder/environments.py	87.33% <ø> (ø)	⬆️
readthedocs/restapi/serializers.py	97.43% <ø> (-0.1%)	⬇️
readthedocs/projects/models.py	85.44% <100%> (+0.34%)	⬆️
readthedocs/projects/admin.py	91.5% <100%> (+0.42%)	⬆️
readthedocs/projects/tasks.py	71.02% <100%> (+0.46%)	⬆️

[agjohnson]

Tests look great! 💯

I'm not sure any of this feedback is necessary, but opened up for discussion at least.

[agjohnson]

🎉