Changed datepicker for flatpicker

[rjlanari]

*Replace the old datepicker with flatpickr and connect the flatpickr instances (if there are multiple ones, e.g. for module phases).

• [x] PR name contains story or task reference
• [ ] Documentation (docs and inline)
• [x] Tests (including n+1 and django_assert_num_queries where applicable)
• [x] Changelog

[m4ra]

@rjlanari @goapunk the flatpicker in mB works as dropdown, while the one here is similar to datepicker. from mB dev:

from this a+ PR:

[rjlanari]

@rjlanari @goapunk the flatpicker in mB works as dropdown, while the one here is similar to datepicker. from mB dev:

from this a+ PR:

Hello @mara, why can it be that when I test it locally I get it as dropdown?