Make it possible to edit existing absence periods

[jonandernovella]

Related issue(s) and PR(s)

This PR closes #654

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

List of changes made

• Added new provider for editing absence period dialogs
• Reused the ModalDialog in the newly created provider
• Make it possible to truly edit time entry ranges

Screenshot of the fix

Testing

• Click on the edit button of a absence range in the absence page and update the from and to dates.

[kusalananda]

• I can not see my own planned absence in the view's lower half graph:
  

• The dates are not shown in the dialogue the first time a planned absence is edited. They are shown on subsequent edits.
  First edit:
  

  Second (and subsequent) edit:
  

• A note should be added (on the page or in the help text) saying exactly what ranges the planner is showing.

• Help text is missing.

The main issue at the moment is that the dates for the planned absence is not shown in the dialogue the first time the user wants to edit the date range.

The adding, editing, and deleting seem to work well!

[ghost]

When I click "edit", I expect the dates to appear in the date fields for planning a new absence (including date pickers) instead of getting a new dialog that works differently.

[kusalananda]

Looks good now. The main issue that I found in my first review has been handled.

[kusalananda]

I'm merging this with the understanding that a help text will be added later.