Extend role type contributor interface so that it can be checked if roles are assignable

• Type: Task
• Status: In Review
• Priority: Minor
• Resolution: Unresolved
• Affects Version/s: None
• Fix Version/s: None
• Component/s: User Management > Roles
• Labels:
  • pt-app-security-review
• Sprint: LIMA | #17 | Apr14-May05
• Git Pull Request: https://github.com/drewbrokke/liferay-portal/pull/1031

There are roles (like Site Admin) that cannot have users assigned to it manually. We need to reflect this in the RoleTypeContributor interface so that new role types can behave like builtin ones.