Success emails

[marioncottard]

Fixes #559

• Added functionality to send email when an execution finishes successfully.

• Refactored functionality to send email when an execution fails.

• Configured rostering as an example.

The recipients of the emails are configured through environment variables

[codecov]

Codecov Report

Attention: Patch coverage is 46.93878% with 26 lines in your changes missing coverage. Please review.

Project coverage is 75.25%. Comparing base (a34a01d) to head (076cbed).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #572      +/-   ##
===========================================
- Coverage    75.29%   75.25%   -0.04%     
===========================================
  Files          258      258              
  Lines        14430    14466      +36     
  Branches      2078     2002      -76     
===========================================
+ Hits         10865    10887      +22     
- Misses        3104     3119      +15     
+ Partials       461      460       -1     

Flag	Coverage Δ
client-tests	77.25% <45.45%> (-0.47%)	⬇️
dags-tests	77.77% <50.00%> (+0.02%)	⬆️

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Components	Coverage Δ
server	77.89% <ø> (ø)
client	77.25% <45.45%> (-0.47%)	⬇️
dags	77.77% <50.00%> (+0.02%)	⬆️

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