Updates the legacy membership providers and WebSecurity/Password changer to stay backwards compatible with their previous settings whilst still keeping all of the provider fixes.
Merge remote-tracking branch 'origin/6.2.0-membershipprovider' into 7.0.0

Conflicts:
	src/Umbraco.Web.UI/umbraco/controls/passwordChanger.ascx
	src/Umbraco.Web/Security/WebSecurity.cs
	src/umbraco.providers/UsersMembershipProvider.cs
	src/umbraco.providers/members/MembersMembershipProvider.cs
Adds notification for 404 errors, updates password changer to use new provider property for legacy backwards compatibility. Fixes validation logic on post.