Passing user objects to MembershipManager methods is deprecated

[pfrenssen]

In #541 the methods in MembershipManagerInterface that deal with users have had their signatures changed to accept user IDs rather than full AccountInterface objects. A backwards compatibility layer was kept to give users the chance to update their code base without breaking anything.

We need to remove the B/C layer and type hint the $user_id parameters to integers.

[pfrenssen]

While testing #541 it was discovered that the Entity API often returns the user ID as a string value, even though it is declared as an integer in the base field definitions. Let's keep the documentation that the user ID should be an integer but not enforce it through a scalar type hint until core is more strict about its data types.