POULPE-521 Case-insensitive authentication in Poulpe

Required behavior:

- In case username is unique
-- If user logs in via UI, it's not important what case of letters he uses
-- If we get info about the user via REST service, then it's not important what case is specified in the GET request
- In case there are already 2 or more users with the same username that differs only in letter case
- Then authentication is case-sensitive and exact match of username is expected
- While authentication via REST, the user information should be returned in response in exact form it's kept in Poulpe DB (the case of letters is returned as is)