Implement backend validation for formio datetime component

[sergei-maertens]

Partly closes #72

Changes

Added serializer field for min/max datetime validation of formio datetime components.

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • Checked copying a form
  • Checked import/export of a form
  • Config checks in the configuration overview admin page
  • Problem detection in the admin email digest is handled

• Release management

  • I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • Ran ./bin/makemessages.sh
  • Ran ./bin/compilemessages_js.sh

• Commit hygiene

  • Commit messages refer to the relevant Github issue
  • Commit messages explain the "why" of change, not the how

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.26%. Comparing base (9232522) to head (78a017d).
Report is 732 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4311   +/-   ##
=======================================
  Coverage   96.26%   96.26%           
=======================================
  Files         731      731           
  Lines       23684    23716   +32     
  Branches     2789     2795    +6     
=======================================
+ Hits        22799    22831   +32     
  Misses        616      616           
  Partials      269      269           

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[vaszig]

Apart from the merge conflicts looks ok to me.