Makefile: Remove `superuser` command, move it to `dev_database`
Makefile: Remove `superuser` command, move it to `dev_database`

`superuser` command used to create a superuser `admin` with password
`admin`. While very helpful for development purposes, it was definitely
a risky business to accidentaly invoke on production server.

That's why the command content was moved to `dev_database` command,
because it's purely for development purposes.

swcarpentry/amy,swcarpentry/amy,pbanaszkiewicz/amy,pbanaszkiewicz/amy,swcarpentry/amy,pbanaszkiewicz/amy