UX: Auto-complete, scrolling and/or other affordances for the Notebook's User filter

Right now, the UX of the Notebook's user filter is based on some ready-built pre-existing components and design patterns that work fairly well when the number of users listed is 1 - 15. In many cases, we'll be dealing with longer lists, and will need a more appropriate UI for filtering against these longer lists.

Auto-complete is one strategy we could consider. We already have some auto-complete patterns in the app that we could tweak and extend.

There are some subtleties that arise because this a filter, not a search field, as such. I think it's important to guide the user. At the most basic end is making sure we indicate how many users, total, are in the list. More nuanced is, say, the ability to "show all" in some sane way. I'm thinking about how filters get used and how sometimes one wants to see "what's available", not just start typing. i.e. when you don't know yet what you're searching for. I can think of several ways this might be represented...

Also, in the case of instructors, the ability to navigate through results user by user in the grading mode seems helpful. I don't know if there's anything we can adopt here that would emulate that experience (one user, then the "next" user) or if it would be desirable, but I thought I'd bring it up.

I forgot to mention: we need to be extra thoughtful about responsive design on this one.