Add configuration for time resolution rounding
Add private method for the TimeEntry model to round time

The method uses the configuration to determine the resolution and is
inspired by the following StackOverflow answer:
https://stackoverflow.com/a/4182700.
Round start and end time before validation (and thus saving)

I've chosen to connect the callback to before_validation because
at a later point we want to check for overlap between time entries
and this should be done using the corrected versions.
Merge branch 'feature/9f7883-rounding-to-time-resolution' into development (closes: #9f7883)
Add some changelog entries for already finished features