Show a connectivity summary&warning timeline on the proctoring page

[blerner]

While proctoring, in the glorious case when students are connecting just fine, working just fine, and have no questions to ask, the proctoring page is quiet. Too quiet, eerily quiet. It's hard for the proctor to know if something has gone wrong, because it looks just like everyone has fallen off and lost connection.

I'm imagining something like a snapshots-over-time line graph, showing how many snapshots have been recorded in the last N seconds, and a warning showing the names of any student both (a) is not finalized and (b) has not had a snapshot in the last N seconds.

[blerner]

Also note that this might mean we need to store snapshots that don't modify any answers, so that we know the connection was still live.