[qa] Enforce conventional commit style

[nemesifier]

Read OpenWISP Commit conventions: openwisp/openwisp-docs#109

I still think we should keep consistency with what we've done until now, so I would use "feature" instead of "feat.
The prefix can be in square brackets, eg: [feature], [change].
Ensure first letter after ] is capital letter.
We should use the same keywords of conventional commits, with the exception of "feat" which is better spelt "feature", 3 characters are worth it in my opinion in this case.
We should also add [change] to the allowed prefixes.

We could then add a commit message template like this: https://thoughtbot.com/blog/better-commit-messages-with-a-gitmessage-template we can try it here first and then replicate it in the other modules.

Maybe we could ditch our own commit check solution in favor of a standard one, I just found this tool which looks really interesting!
https://commitizen-tools.github.io/commitizen/

This change will help us to generate change logs more easily later on.

[devkapilbansal]

@nemesisdesign can we create pre-commit hooks too to ensure that basic checks works correctly before making a commit. Occasionally new contributors forget to run openwisp-qa-format or following flake guidelines due to which build failed.

[nemesifier]

@devkapilbansal I won't be able to help with this now, it's useful to do it but not more important than releasing new features and helping the community to use the work we've been doing, for these reasons I'm urging all the contributors to focus on the issues in https://github.com/orgs/openwisp/projects/4 at least until we have released openwisp-controller 0.8, openwisp-monitoring 0.1, openwisp-firmware-upgrader 0.1 and openwisp-ipam 0.2.