Login against Postgres DB is case-sensitive

[Al2Klimov]

Describe the bug

@yhabteab just confirmed that, with users in a Postgres DB, one can't login as "ICINGAADMIN". (Admittedly there's a LOWER() unique index enforcing distinct case-insensitive user names.)

Expected behavior

Just like in MySQL, icingaadmin can log in as "ICINGAADMIN", via either:

• LOWER() in query
• citext

Additional context

Icinga/icinga-notifications#203 (comment)