Progress indicator for build summary page

[mikepurvis]

Currently the build summary page shows a list of the built and currently-building steps, but there's no indication at all of how far along things are. At the very least, it would be great if this page could give information comparable to what is shown by nix build on the command line— x/yyy downloaded, y/zzz built, etc.

But an even better version, though obviously more complex, might be if it could produce an actual ETA based on looking at timing info from previous runs of the same derivations (could be as simple as just mapping by name/pname) and then it would be "here's the graph of remaining things to build, here's the hot path through it all, so assuming perfect parallelism, time remaining is mm:ss. Maybe this would be something to look at implementing first in the command line tool and making Hydra simply expose the information once available, rather than building the logic for it directly into Hydra.