Use activity labels to determine billability-excluded hours

[LucaScorpion]

You can create a label that applies to an activity:

This mechanism can be used to add a label to activities that should not be included in the billability calculation, so that the activity names don't have to be hardcoded.

[LucaScorpion]

While there is an endpoint to get the tasks with their applied labels, this endpoint doesn't seem to work for the standard "User" role, making this change significantly less usable. So we should probably reconsider the approach here.