Pass schedules to emailer cronjobs from values.yaml

[Redande]

We want to run prague and purkyne emailers multiple times a year, but the dates vary too much for cronjob's schedule syntax. Added the dates to values.yaml so that we can run the same template multiple times a year, and to easily modify when to run them.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.43%. Comparing base (45b3ffb) to head (a4258df).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1263   +/-   ##
=======================================
  Coverage   66.43%   66.43%           
=======================================
  Files         144      144           
  Lines        6042     6042           
  Branches     1434     1434           
=======================================
  Hits         4014     4014           
  Misses       1889     1889           
  Partials      139      139           

Flag	Coverage Δ
backend	66.43% <ø> (ø)

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