Synchronize customer and assigned users for task

[neferin12]

For every task synchronize the additional fields customer and assignedUsers.

Example:

[
  {
    "id": "someId",
    "title": "someTitle",
    "note": false,
    "customer": "someCustomer",
    "assignedUsers": ["someUser", "anotherUser"]
  }
]

customer can be an arbitrary string that will be shown to the user.
The strings in assignedUsers need to be the content of the sub claim in the oicd token.

ToDos:

• Implement in TimeClicker backend
• @kraeks Change Kanban Board accordingly