Pass a filter parameter to the MessageView model

MessageView itself doesn't set it yet.
Manage state for empty message lists more explicitly

We'll need this if you switch filters in that state.
Move findTopMessage and findBottomMessage back into MessageView

Sigh. Shouldn't have moved them out.
Blegh. Filter's keys are identical to input

So they round-trip and I can use the same filter.js file on client and server.
Implement MessageView#changeFilter

It works!!
First version of filter support!!!!

IT WORKS!!!!