Cache hover card responses

[matthew-white]

From #1067 (review):

What do you think of caching the response of hover requests until user navigates to another page/route? right now every time I hover on a link a request is sent.

I think this is a good idea. It's very possible that the user will hover over one resource, then move somewhere else, then return to hovering over the resource.

One thought I had is that we should consider putting a limit on the size of this cache. If the user has hovered over a bunch of resources on a single large page, we might not want to keep them all in memory. Maybe just the last 10 or 20 that the user has hovered over.