MembershipManager::createMembership() is accepting invalid account objects

[pfrenssen]

MembershipManagerInterface::createMembership() is accepting objects of the type AccountInterface for the $user parameter. In the method it will then pass this to EntityOwnerInterface::setOwner() which only accepts UserInterface objects.

This means if we pass in an object that implements AccountInterface but not UserInterface then we will get a fatal error. This might happen for example by passing in an AccountProxy object or an AnonymousUserSession.

Scope of this ticket:

• Change the signature of MembershipManagerInterface::createMembership() to accept UserInterface objects instead of AccountInterface objects.
• Update the implementation at MembershipManager::createMembership().

[pfrenssen]

Fixed in #603