Add check variables in constants to hide in add variable popups in nurseries and trials,
Use new standard variable objects in the html pages instead of hardcoded ones,
Add/update check variables if checks are selected and delete check variables if checks are cleared,
Retrieve saved check variables when editing a nursery and when using a previous nursery as a template,
Disable check variable fields when editing a nursery without clicking the Replace button first

GCP-9879

Reviewer: Aldrin Batac
Prepare test data for unit tests of check variables

GCP-9879

Reviewer: Aldrin Batac
Add unit tests for saving and retrieval of check variables

GCP-9879

Reviewer: Aldrin Batac