Fix EZP-21703: RoleService does not validate Limitations in assignRoleToUser() and assignRoleToUserGroup() methods

This PR resolves issue https://jira.ez.no/browse/EZP-21703

Validation of `RoleLimitation` is implemented in `RoleService`.
`SubtreeLimitationType::validate()` is improved by comparing complete path string.

Unit tests for methods `assignRoleToUser()` and `assignRoleToUserGroup()` in `RoleService` are implemented.