paging support

[karenetheridge]

Support ?page_num=X&page_size=Y parameters for (some) GET requests that would ordinarily return a lot of content. Enforce a reasonable max page size. Possibly should allow caller to specify which column to sort on so the paging comes out as desired.

Prime candidates:

GET /user
GET /build/*/device
GET /build/*/rack

Additionally, the Accept-Range and Content-Range response headers should always be used to describe the parameters of the content (and 416 Range Not Satisfiable, when appropriate).

alternatively, use the Range header to request pages and to negotiate content size. see https://tools.ietf.org/html/rfc7233, in which case we can return 206 Partial Content instead of 200 OK responses.