edit a planned vacation period

[HenrikeW]

It should be possible to change the start and end date of a planned vacation period.
No other changes to that period should be possible, i.e. you can't add gaps into the middle of a vacation period.

[HenrikeW]

UI: There should be one edit button per row in the table. It should either open one dialogue that gives the user the option to edit both start and end date. Or it should turn the dates into datepickers that can open one dialogue each. Whatever is easiest to implement.
We need to make sure that only valid new dates can be save: start date before end date, no other time entries during the chosen period.