Add "target date" to action item / bug creation forms (PMT #95770)

PMT #95770

Target date will default to the milestone's target date if it's not
provided.

Includes a temp patch to bootstrap-datepicker z-index until this pull
request is merged:
https://github.com/eternicode/bootstrap-datepicker/pull/876

Adds a test for valid estimated time on Project.add_item