Create user from string with username generated in backend

[reiterl]

Resolve #2921

In this pullrequest I have slightly changed the createFromString function, to allow the backend to generate the username.

[Elblinator]

the User which is created should be inactive

currently the created account/participant is active, please change this to inactive

after a user is created a snackbar should open which says: "A user with the Surname 'bla bla' and the username 'blabla' was created"

this is currently missing, please add

[reiterl]

I have added code for set inactive and opening the snackbar.

[Elblinator]

given that the name is saved in first and last or only the first name:
Please change the snackbar message from the last name/ Surname to the first name

[reiterl]

Changed the snackbar message to use first name instead of last name.

[Elblinator]

function