Add MAILMAN_WEB_SOCIAL_AUTH for social logins
The `MAILMAN_WEB_SOCIAL_AUTH` list contains a default set of social
login provides. This was previously included in `INSTALLED_APPS`.
Separating it to it's own list makes it easier to disable or otherwise
override which social login providers are enabled without the need to
modify `INSTALLED_APPS`.
Older installations where `INSTALLED_APPS` were overridden continues to
work. The `MAILMAN_WEB_SOCIAL_AUTH` is ignored for those and
`INSTALLED_APPS` is used as is.
Replace them with:
* containers' hostnames
* gateway's IP address for default SMTP_HOST
* Core: when SMTP_HOST undef, echo the default value
* docker-compose: add port mapping
* docker-compose: drop network driver conf
* Exim macro: use localhost as LMTP host
* Update README.md
* docker-compose: Limit port mapping to loopback
* Update other docker-compose example files
* Remove the secret key.
* Use the latest tag for postorius.
* Use the latest django-mailman3 that supports Python 3.
* Use minimum version on Django-mailman3.
* Fix import errors.
