Revamp core calculations in issue export

[maximilianruesch]

Changes some core behaviour in the issue export, namely:

• Changes the working days calculation to just be a diff from the start and end date
• Excludes weekends from this diff
• Precomputes the start and end dates and changes the date filter to only include issues if their first in-progress transition is after the start date and their last done-transition is before the end-date.
• Only shows "done" status for selection on what to include in the export

Open questions:

• What should happen with issues that never had a in-progress transition? Currently they are filtered out. -> Filtered out is fine.

• Closes: https://projectcanvas.atlassian.net/browse/SCRUM-104