Schema: make columns w/ IW2 user names case-insensitive(?)

[Al2Klimov]

I was wrong (Icinga/icinga-notifications#203 (comment)). Icinga Web 2 user names are also (kinda) case-insensitive in Postgres. CC @lippserd

But here we have some columns with some kind of user names. Some of them even explicitly "reference to web user". I can only guess, but I guess some should be case-insensitive, just like Icinga Web 2 user names themselves. via the new, smooth citext.

[Al2Klimov]

CC Icinga/icinga-notifications#71