database creation/migration script should create separate user for database

Currently it seems that pleroma always uses postgres user declared in config/dev.exs which is superuser for a postgres instance, for security and convenience reasons it should use separate user that only has access to pleroma database.

currenlty psql commands to create user look like this:

create user pleroma;
alter user pleroma with encrypted password '<your password>';
grant ALL ON ALL tables in schema public TO pleroma;

(sorry for bad capitalisation)

install\migrate scripts probably should ask for username/password or store it separately, because creating a database and user would require superuser access.