Enabled Materials Manager for Scheduler

Hi @ikeough, apparently we can keep ```MaterialsManager``` in ```RevitServices``` and just make sure ```InitializeForActiveDocumentOnIdle``` method is called from within an idle thread. Since this method is only called from ```RevitDynamoModel``` we can always ensure it is the case (although we should still think deeper about the issues around Separation of Concern we have with singleton classes such as ```MaterialsManager```).

Note that the new changes can be compiled when the scheduler is disabled, which means if we ever choose to do that prior to release, this will require no code changes.