Cache server-side, external API requests

[antidipyramid]

There are a few places where we're making requests to Granicus on behalf of users (e.g. to verify meeting media links or get running events). These should all be cached so we're not making these requests to Granicus for every single user.

The homepage in particular slows down (>500 ms) on meeting days because of multiple Granicus requests. Just a 60 second server-side cache would speed up the homepage for the vast majority of requests.

We added some explicit caching for running events requests in #1111. For code clarity, it's probably a good idea to do this caching in the timed_get function so we're not having to manually set/clear the cache for every request.