Adds AddRelation and missing events in RelationService from v7 into v6
Ports behaviour from membershiphelper from v7 to 6 to make them the same
Fixes: Password is not saved when using the Register snippet
Merge branch '6.2.0' of https://github.com/umbraco/Umbraco-CMS into 7.1.0

Conflicts:
	src/Umbraco.Core/Services/IRelationService.cs
	src/Umbraco.Web/Controllers/UmbRegisterController.cs
Merge branch '7.1.0' of https://github.com/umbraco/Umbraco-CMS into 7.1.0