Fix "+ create User" action in participant search fields

[Elblinator]

Current behaviour:
If you search for a user in a participant search field and the participant is not in the participant-list then it is possible to run into one Error: Error: A user with the username <name> already exists

Reproduction:

1. create the account Firstname: xx, Surname xx; automatically will create username: xxxx
2. navigate into a meeting > motion > LoS
3. Search for a participant xxxx
4. you should get the option to "+ create User"
5. if you activate the option the Error is thrown: Error: A user with the username xxxx already exists

Wanted behaviour:

1. the input field: Select or search new speaker ... should send the whole string as the surname
   If you have the input Kim Meier and create a new user, then the created account should have no first name and the surname Kim Meier
2. the User which is created should be inactive
3. after a user is created a snackbar should open which says: "A user with the Surname '' and the username '' was created"

[reiterl]

The createNewSelectedUser uses the userRepo.createFromString function.
A possible solution is to clean the parsed username in this function and use the backend generate code to set the username. As far as I know, this was the solution, we want to go for.