Collation specialization appears to be out of date

[eemeli]

We currently build our own postgres docker image and do extra steps during test setup to help ensure that we have a tr_tr collation available in Postgres. This was added to resolve this bug in 2017.

Reviewing the situation now, it looks like the same behaviour is available "out of the box" with our current Django and Postgres versions by using the tr-x-icu collation. We should despecialize what we're doing and use that (and other ICU-backed locales) where appropriate.