Feature/pmfloodcontrol

Adds flood control & max recipient option to Conversations.

To add flood control, I changed the inheritance of the two models to have a common parent. I elected to not add a settings page for changing the flood control defaults; no one ever messes with them in Vanilla, let alone Conversations.

Max recipients uncovered a few issues with tokeninput I needed to work around.  I'm not super pleased with the AddUserAllowed method definition but couldn't think of a better way to allow both use cases (sometimes we have a recipient count and no ConversationID; sometimes the inverse).