Fixes and improvements to make the role work better.

[rivimey]

• I have created a more useful README;
• I have fixed some dependencies which had become stale, and added a few that were missing;
• The role has been made more flexible, by enabling MM3-Core to be installed independently of Postoorius or Hyperkitty
• Some issues with the configuration have been addressed and more config items added to make it clearer.

This role is still a work in progress, but it is much better than it was.

[natefoo]

Oh wow, I had not even noticed that anyone else was using this and that you'd done all of this excellent work. I just got done overhauling it and went to PR my changes and found your PR.

My changes are here: https://github.com/galaxyproject/ansible-mailman3/tree/update-2022

I will try to integrate yours so that you don't have to update everything for the inevitable conflicts mine creates. Sorry I missed this until now.