Easy override for social logins (#446)

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.

web/README.md

@@ -71,6 +71,14 @@ change them unless you know what you want.
   `http://mailman-web:8000` by default so that Core can fetch templates from
   Web.
 
+- `MAILMAN_WEB_SOCIAL_AUTH`: This is a list of Social login providers.
+  It contains a default set of providers. Override it if you want to remove
+  or disable social login entierly.
+  If `INSTALLED_APPS` is overridden `MAILMAN_WEB_SOCIAL_AUTH` is not used and
+  you must specify any social login provider in `INSTALLED_APPS` instead.
+  See [settings.py](mailman-web/settings.py) for implementation details.
+
+
 Running
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