[Organizations] Let company owners also be an employee to another company.

[TiSiE]

Currently, an user can be either the owner of a company or an employee.

There might be cases, in which a company owner might want or need to be an employee (or even owner) of another company. So we should provide the possibility of doing that.

There are however a few things to consider (this list is not complete):

• We must specify how to organize the user/company relations. (Currently only one company per user is allowed)
• To prevent permission issues, the user MUST choose, for which of his associated companies he want to work in the current session.
  ** Selecting via the user menu
  ** Every permission check must be aware of the chosen company in the session.