Clear NA entry when logging time

Some employees create a NA entry for each day of the month at the beginning of the month. This way they don't have to enter anything on days where they don't work. On days when they work, they replace the NA entry with their actual work entry.

To replicate this behavior, we should check, when logging time, if there is already a NA entry and if so delete it.

This is a breaking behavior and should probably be configurable as part of user preferences/settings.