Separate common parts of `rails`/`rails-pgsql` in common

[JedMeister]

In common there are 2 rails build scripts, one that most apps use which uses mysql/mariadb as a DB backend, as is likely obvious rails-pgsql uses postgres as a DB backend.

They share a bit of code, so It seems to me that it would make sense to pull out the parts common to both scripts into a rails-common (or similar).