Fixes a subtle bug in the user search ranking where a case-sensitive
match (PostgreSQL string equality) was used on a value that had
previously been converted to lowercase. As a result, exact matches could
receive the lowest priority instead of the higest priority.

For distinctive usernames, this was not a problem, as those would result
in unique matches anyway. However, when the username matched a prefix of
a common human name, an exact match could get buried at the bottom of
the results.

For example, since Alexander is a very common name, this made it
virtually impossible to find @Alexand in most GitLab dropdowns.

Changelog: fixed