[qa] Simplify migration checks

We currently have two migration checks:

• one checks whether there are pending migrations to create (./manage.py makemigrations --dry-run)
• one checks whether the names of the migration files have auto in them

The problem is that we have to specify both --migration-module and --migration-path.
I think we can avoid doing this and have only one command line flag.

I would get rid of --migration-path, keeping only --migration-module, although I would rename this flag to --migration-modules while keeping backward compatibility with the old flag.

--migration-path should be ignored (printing a deprecation warning).

We will need to upgrade the modules using this: https://github.com/search?q=org%3Aopenwisp+%27--migration-path%27&type=Code

Start with the migration-check entry point invoked through ./manage.py, then search the repository and linked OpenWisp modules for --migration-path and --migration-module. Done means one plural --migration-modules flag is supported with backward compatibility, --migration-path is ignored with a deprecation warning, and the affected modules no longer require both flags.