This way it won't be possible to browse all user URLs by just going to
/users/1, /users/2, /users/3, ... and collect usernames, which might not
be desirable in some cases.
Note we could use the username as a URL parameter and just find the user
with `@user = User.find_by!(id: id, username: username)`, but since
usernames might contain strange characters, this might lead to
strange/ugly URLs.
Finally, note we're using `username.to_s` in order to cover the case
where the username is `nil` (as is the case with erased users).
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