Refactor invitation/reminder code

[PhantomWatson]

• Invitation/reminder code is split up between the client and admin versions of the SurveyController and there's a bunch of redundancy.
• SurveyProcessingComponent is pretty vague. Some code should be split off into InvitationComponent and the rest of the code should be under a more specific-purpose class.
• SurveyProcessingComponent::getApprovedCount() looks like it could be replaced with a one-line call to a method of Hash