Regression when checking out repositories for a pull request

[jonasraoni]

The checkout_user_repo.sh has regressions when compared to the startSubmodulesTRAVIS.sh

1. It's limited to the pkp-lib, but it should cover all submodules
2. It can't process long branches (e.g. feature/stable-3_4_0/1234-fix-issue)

As part of this task, I think we should try to drop the ##user/repo##" syntax, and do the checkout automatically (if we don't have the right commit in the PKP repository, then check if the user/submodule has what we need).