fix(notifications): set timestamp for sent emails

[RadovanTomik]

Negotiator pull request:

Description:

Sets timestamp during email sending.

Checklist:

Make sure you tick all the boxes bellow if they are true or do not apply:

• I have performed a self review of my code
• My code follows Google Java Code style
• I have made my code as simple as possible
• I have added unit tests and the code coverage has not decreased
• I have updated the documentation in all relevant places

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.41%. Comparing base (abb755c) to head (6077fae).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #321      +/-   ##
============================================
+ Coverage     79.39%   79.41%   +0.01%     
  Complexity     1091     1091              
============================================
  Files           147      147              
  Lines          3281     3284       +3     
  Branches        179      180       +1     
============================================
+ Hits           2605     2608       +3     
  Misses          552      552              
  Partials        124      124              

Flag	Coverage Δ
unit	79.41% <100.00%> (+0.01%)	⬆️

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