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
PrevNextWeekMixin doesn't need to inherit from View
Add ReportFileGenerator

Generalized report generation to work with different reports.
use queryparams instead of hardcoded urls for report exports
Add export feature to staff report and weekly summary
Merge pull request #112 from nikolas/target-date-on-create-action-item

Add "target date" to action item / bug creation forms (PMT #95770)
Merge pull request #114 from nikolas/spreadsheet-exports

Spreadsheet exports (PMT #95661)