Add function for editing absence ranges

[jonandernovella]

Related issue(s) and PR(s)

This PR closes #599

We still need the UI component for specifying the new ranges, which could be another issue.

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

• New function which allows updating existing absence ranges
• Added better error handling

Testing

• Test creating, editing and removing
• At the moment, the edit button will update the absence range to be as of the date of the current day

[KattisLej]

Starting review

[KattisLej]

Apart from the comment- checking so the date is not already reported for
I think the code and implementation looks good.

[KattisLej]

Well done