Figure out cache-control for our REST API endpoints

[humphd]

According to webhint, we aren't setting proper cache-control headers for our REST API endpoints. This means that users' browsers aren't able to cache these responses effectively.

Here are the endpoints we have, and some notes on each:

• /admin - areas only available to admin users. Not critical for efficiency
• /auth our login/logout routes. I can't imagine we'd want to cache these
• /feed various feeds we offer of our content. These could probably be cached for an hour
• /feeds various methods for working with one or more feeds. These could be cached for minutes to an hour?
• /health healthcheck, and version info. Probably could be cached 0 to a few minutes?
• /planet the old planet we have hidden in our app, could be cached for minutes to an hour?
• /query I don't think this should get cached
• /posts this is probably one of the heaviest accessed routes, worth thinking about this carefully. A post can probably be cached for an hour or less. The list of posts could be cached too, not sure how long.
• /sp our metadata for SSO. This can get cached for a long time
• /stats could get cached for an hour or less?
• /user/info is the data for a logged in user, so we don't want to cache this I don't think. Probably the same with /user/feeds

[humphd]

Closing in favour of #1936.