Create objectId domain / data type in schema

[frcroth]

The objectId columns could be moved to a new domain that enforces length / chars.

I tried this in #8372, but I had problems with Slick, since it did not recognize the new data type as string, but rather as BLOB.

          Alright then I’ll have to decide! Please have a look if there may be significant downsides to this, and if not, implement it :)

Originally posted by @fm3 in #8372 (comment)

[fm3]

Since the standard slick connection couldn’t handle this, I think it’s not really worth investing further in this just for the benefit of a more readable schema.sql