* Allow single letter user id.

They do work - I use one and login using Authelia, but then I am not able to update the profile to update avatar or add comment because "Invalid username: r" errors

* Add test for the util with uid matcher

* Move the test to separate directory

* Use goimports on the test file

* Rename the test for a mroe matching name, add failing test

* Update the regexp so that two letter work

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Co-authored-by: Roman Kamyk <roman@kamyk.me>