Modifications for background query execution

[bruth]

See chop-dbhi/serrano#204 and chop-dbhi/avocado#231 for background.

There is one primary internal change and one primary UI addition that must be made to accommodate the new Serrano resources.

The internal change is to use the new/changed query endpoint to explicitly POST the context and view to submit the job followed by polling the returned job URL until the result is ready. In normal circumstances this interaction will be negligible from a performance standpoint, but the interaction is simply different.

The UI addition is a prompt for long-running queries for choosing to wait for or cancel the query (after some configurable amount of time). If the query is cancelled the results page will be convey this using some message, otherwise the polling will continue.