Allow caching some API responses that change rarely

[cpeel]

Allow the browser to cache some API responses for endpoints that change very very rarely. These in particular are used by the new PI and letting the browser cache them for a bit will make the initial PI load faster when users are coming in and out of the PI (the PI doesn't reload these when it moves between pages).

We cache the site documents and dictionaries for a day. Project pickersets for an hour.

Sandbox: https://www.pgdp.org/~cpeel/c.branch/cache-some-api-responses/

Testing is making sure the Cache-Control header is included in the response, eg:

export API_KEY=review
curl -i -H "Accept: application/json" -H "X-API-KEY: $API_KEY" \
    -X GET "https://www.pgdp.org/~cpeel/c.branch/cache-some-api-responses/api/v1/documents"