Add a treasury tip slashes button

[rrtti]

Add a button, on the treasury tips dashboard to slash a tip if needed by a Council member - A suggestion would be to position it bewteen Cancel and Tip buttons.

[wirednkod]

@rrtti What would be the action of "slashing" a tip? can you elaborate please?

[rrtti]

@wirednkod a new call on tips pallet was added with the last runtime upgrade: slash_tip - Remove and slash an already-open tip. This call allows us to deal with any malicious behaviour on this side of treasury. In this direction, any councillor should be able to open a proposal to slash an open tip and findthe button to do so easily on the tips window in Apps.