Better Circuit lookups: by device, etc.

We have the ability to look up Circuit object by their endpoints via the Interface primary keys or natural keys (interface name slug), but it would be also useful to be able to look up Circuits directly by device hostname. This is possible via the `circuits` detail view on Device, but being able to do this directly on Circuit would be nice.

Something else along these lines that would be awesome would be the ability to look up mutual Circuits between two devices. Like, given two device primary/natural keys, return every Circuit object that connects the two.