Fix assignment.update if candidate is already on LoS

[bspekker]

Description
If candidate is already on LoS, candidate is not added to LoS when phase is changed to voting/in the election process.

Reproduction

1. in settings > elections: enabe 'Put all candidates on the list of speakers'
2. create an election
3. add candidate A and B to list of candidates
4. add candidate A to LoS
5. change phase of election to 'in the election process' --> Error: User 5316 is already on the list of speakers. and no user is added to LoS and phase is not changed

Payload

[
  {
    "action": "assignment.update",
    "data": [
      {
        "id": 338,
        "phase": "voting"
      }
    ]
  }
]

Response

{
    "success": false,
    "message": "User 5316 is already on the list of speakers.",
    "action_error_index": 0,
    "action_data_error_index": 0
}

Expected behaviour
When changing the phase of the election, candidates that are not already on the LoS should be added and phase should be changed to 'in the election process'/voting.
(Candidates that are already on the LoS should be ignored)