Allow users to cancel pending trades

[lukechilds]

We can now do this with:

_api.debug({method: 'cancel', uuid: "c233ab1ebbb5e4b34341779923af19506f5714c512661394f5ea28c4d822a155"})

https://github.com/jl777/SuperNET/commit/015b4cec17a75154bd4ef5f3bc0ab907284a8588

(at time of writing not yet in a marketmaker release version)

[lukechilds]

This works when a newly placed swap is pending but is unpredictable in other scenarios. There is no way for us to handle this and accurately tell the user what happened with the current mm implementation so we'll need to wait until marketmaker v2 to implement more reliable user feedback for the cancel button.