Case-insensitive authentication in JCommune via Poulpe

As a User I'd like to be able to log in to JCommune without warring about case of the letters so that it's easier to log in.

Acceptance Criteria

• This process is already implemented in JCommune, but not in case when Poulpe is involved. So only Poulpe-related stuff should change, the rest should stay the same.
• If there is a single user in JC, but authentication failed, then request to Poulpe is send for authentication with exact data as was input in the log in form.
  • If user was authenticated successfully by Poulpe and the user in the response is exactly the same as in internal DB, then its password should be updated in JC DB
  • If user was authenticated successfully by Poulpe but the returned user is in different case, then JC should store additional user with different case in username.