Add "target date" to action item / bug creation forms

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
Merge pull request #112 from nikolas/target-date-on-create-action-item

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