Update a unikernel from our reproducible build database

[hannesm]

In albatross-client, we support the update command, which (a) fetches from builds.robur.coop the current unikernel job, (b) looks whether there's a more recent build of the same job, and (c) displays a url to the diff, finally (d) deploys the new unikernel

For mollymawk, we should have a two-stage interface where we can click on update, and then the diff is displayed inline, and we can confirm or reject the update.

[PizieDust]

@hannesm I see that builder-web uses dream. Do you think it's a great idea to add two new endpoints that just return json data. I think this will be great for mollymawk, this way we just get the data and display it however we want.
The endpoints I propose should be:

GET .../latest/uuid -> return the uuid of the latest binary

{
   "uuid": "44bfafe0-97a4-471d-b96d-be0dd887d0ef"
}

GET .../compare/json/old_uuid/new_uuid -> returns differences formatted in json

Let me know what you think.