Allow returning sticky posts at top of posts collection #2764

This idea is an enhancement proposal that evolved from discussion around #2210.

The current solution for sticky posts is to not give them special treatment by default, but to provide a ?sticky= parameter to explicitly exclude them or to return only stickies, added in #2708.

@aaronjorbin proposed revisiting a way to replicate WordPress theme behavior within the API posts collection, .i.e. return stickies at the top of the collection first and then return normal non-sticky posts as normal after. Providing a way to do this in one request would be an improved developer convenience over retrieving sticky posts and non-sticky posts separately in order to render a client application posts list that replicates the normal WordPress theme loop behavior.