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.
47f70d73bd