Mailman3 Integration V2

We've come up with a new plan for integrating mailman3, which is to standup mailman3 on a server of it's own.

New mailman test servers:
lists.preview.boost.org
lists.stage.boost.cppalliance.org

@williln @gregnewman I have added you as a list owners and superusers. If you DM me a SSH public key I can also give you root SSH access on the machines.

There is an open question about how customizations will be implemented. Mailman provides templates to customize emails, and those are maintained in the web UI. Mailman installation was done with https://github.com/cppalliance/ansible-mailman3 which installs nginx, elasticsearch, mailman-core, mailman-web, postfix. If we are going to modify Django, it should be version controlled. Ansible is already writing a django settings.py file, so django config settings (and other files) could be stored in that repository in a boost branch. Yet another idea is to keep a mailman/ subdirectory in https://github.com/cppalliance/temp-site, as a central location, and those files will be remotely deployed/copied to the mailman3 servers.

What customization should be done -

- OAuth2 integration to the main website, as automatic as possible
- Front-end stylesheets, headers. preserving mailman3 features such as search box, active posters, etc.

Review cppalliance/ansible-mailman3 and cppalliance/temp-site, then resolve where Django customizations and Mailman templates should live. Define the OAuth2 integration and frontend customization requirements before implementation; done should include agreed, version-controlled changes deployed to the preview and stage Mailman servers.

Written by the indexing model from the issue text.