Resolve "Check username uniqueness without case sensitivity"

What does this MR do?

This MR fixes the issue with usernames that only differ in the use of lower/upper case.

Actions taken to fix the bug

The findByUsername has been replaced with findByUserNameIgnoreCases, this does imply that the users can now also login with any combination of lower/upper cases, e.g. admin can log in with aDmIn as long as the password is still correct. 😛

Does this MR meet the acceptance criteria?

• I have added tests to validate the fix.
• I have updated the documentation accordingly.
• I have added translations for all new client-side text.

Closes #228 (closed)