introduce envsubst for config.js, update deps

[khassel]

This is the implemenation of envsubst discussed in #1756

Documentation update will follow after merge.

[codecov-commenter]

Codecov Report

Merging #3032 (932de0d) into develop (4b478a5) will increase coverage by 0.32%.
The diff coverage is 83.63%.

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@@             Coverage Diff             @@
##           develop    #3032      +/-   ##
===========================================
+ Coverage    22.65%   22.97%   +0.32%     
===========================================
  Files           52       52              
  Lines        11455    11502      +47     
===========================================
+ Hits          2595     2643      +48     
+ Misses        8860     8859       -1     

Impacted Files	Coverage Δ
js/app.js	82.36% <83.63%> (+0.09%)	⬆️
modules/default/updatenotification/node_helper.js	90.27% <0.00%> (+12.50%)	⬆️

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[rejas]

Can you add an example(maybe just with a comment in the sample.config so that people know how to use it?
Would it be possible to add a test for this?
And of course the async question if we convert the whole loadCOnfig mthod to async without callback?