Add clone and checkout clipboard commands to board and changes viewes

[morucci]

trafficstars

• I'd like to be able, when looking at my change board, to click on a button to let Monocle prepare for me the checkout Change git command.
• The same way I'd like to a have button to get the clone command in my clipboard.

It seems to be possible to have the web app build clone and checkout commands from the change URL. However this could be messy and it is probably better to make the Monocle API annotate changes with the provider type to let the web app strictly identify the provider type of a change and compute correctly commands.