add some style to activity list item when they're multiple users on a tab

[aarondruck]

it could look something like this

[ianb]

There's some progress, right now just everyone's name.

We still have to figure out how we want to present it, and we probably want to hide the now-duplicate items.

We should hide viewers (so only the presenter's item is showing), and on live collaboration only the oldest tab.

[aarondruck]

currently we have "with [username]" at the end of the string