[#3049] Fix end-date of plan can precede that of action templates

[linssen814]

Added validation to PlanForm that checks whether the selected end date of a plan does not precede the would-be end-dates of actions in a selected plan template.

Taiga: https://taiga.maykinmedia.nl/project/open-inwoner/issue/3049

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.14%. Comparing base (788a90a) to head (3d082d5).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1630   +/-   ##
========================================
  Coverage    94.13%   94.14%           
========================================
  Files         1088     1088           
  Lines        40150    40181   +31     
========================================
+ Hits         37797    37828   +31     
  Misses        2353     2353           

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[jiromaykin]

Approved - also: when putting sprint PR's in Review, drag them in Taiga out of 'in progress' into the "in review" column,
After the PR is merged with Develop, then you can drag the issue in Taiga to 'Client review'
For loose issues/Userstories this is a bit different (when asking for codereview, first drag it to 'implemented', and after merging drag it to 'Ready for test').
So for sprintPR's inside Userstories there's double the amount of dragging to be done.

[jiromaykin]

@linssen814 👋 Kun je de merge-conflicts oplossen voordat we deze rebasen?
Beter ook: alleen PR's aftakken van opnieuw geupdate Develop branch (elke dag meermaals Git pull) en dan kun je lokaal eerst rebasen/forcepush op Develop en dan lokaal merge-conflicts oplossen, nadat je je laatste verandering hebt gepusht (zonder commit).