MNG-887 - Revise vacations rules
Implements:
- Semi-vacations/slow weeks
- Scoped vacations: reduce availability to/from specific projects, clients or roles
- Revision of notice delays for vacations
- Explicitly allow to call sick days for mental health (including stress, overwork/burn out, etc.) as discussed in https://doist.com/blog/mental-health-and-remote-work/