Add vendor / employment-history related properties/distinctions

[dgor82]

• [ ] There are volunteers without any vendor: verify that in principles, users/roles can exist without any vendor, or decide that there is a general 'Volunteers' vendor so that this is more explicit.
• [ ] Is there a more elegant way than the current use of a random past date for the first employment-history, when we don't care when a user started working for a vendor?
• [ ] There are vendors who are on the festival side (Auftraggeber) vs. vendors who are Auftragnehmer... Think/ask, this surely must make a difference in terms of roles/rights etc.?
  • [ ] At least for now distinguish this as a property of vendor
  • [ ] Check whether this influences roles' rights to submissions or notifications in any way