Remove P0 label from the Contribution readme as it does not exist.

Closes https://github.com/openSUSE/open-build-service/issues/2146

memer -> member

Typo. cofiguration -> configuration

The password for Admin in CONTRIBUTING.md is opensuse and not buildservice.

We can't exec start_test_backend from inside script/ as this
clashes with https://github.com/rails/rails/pull/13528 somehow.

* Rename load_dev_backend to start_development_backend
* drop db:fixtures:obs