new GUI modal: tabulation confirmation page

[artoonie]

closes #679

• Creates new GUI Tabulation Dialog (see corresponding issue)
• Has a check to make sure the # of ballots didn't change between "Check Ballot Count" and "Tabulate."
• Extensible so we can later has the entirety of the CVRs and check that there was no change whatsoever, not just in ballot counts
• Extensible so we can pull additional metadata and show on the GUI before hitting tabulate

[yezr]

Two things I noticed while looking over this

• The progress bar hangs during tabulation. It starts out moving, then at some point during tabulation hangs. Here's a video.
• Each run of tabulate takes a little bit longer than the previous one. Here's some splits I did with the Minneapolis Mayor test data. It got about 5 seconds longer each time I ran it. Not the end of the world, but might be worth looking into...

[artoonie]

Good catch! Both of these are present in 1.3.2 as well, though I agree this PR makes them more evident (especially the GUI lag).

I'll tackle the increase in timing first, then look at the GUI freezing. Since both exist outside of this PR, it may be useful to have separate tickets for them? But not super necessary.

[yezr]

I'll make new tickets. A P0 for the UI hanging #830. A P2 for the increased duration on consecutive runs #831

[artoonie]

Turns out both had the same underlying cause, and both are fixed in #832

[tarheel]

@HEdingfield since this is mostly UI, would you have time to review it?