Prevent creation of lower case versions of existing usernames

[anttimaki]

The old implementation prevented creation of upper case versions of existing usernames, but didn't prevent creation of lower case version of existing uppercase usernames.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.95%. Comparing base (279e50c) to head (6df5946).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1018 +/- ## ======================================= Coverage 92.95% 92.95% ======================================= Files 314 314 Lines 9070 9070 Branches 806 806 ======================================= Hits 8431 8431 Misses 530 530 Partials 109 109 ```

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