Add (back in) support for jumping to lat/lng or panoId in Explore page

[misaugstad]

As part of #4097, I did a big rewrite of much of the front-end code for the Explore page. I didn't include support for starting at a specific pano or lat/lng, because it was a bit of a hidden feature that we don't actually want users to make use of. It's mainly meant as an admin tool on test servers right now.

Half of the infrastructure is still in place for it though, and I want to finish adding it back in. Just ran out of time before pushing out the bulk of the code.

Would also love to fully support this for all users in a way that doesn't break assumptions that the rest of the site has, though that would take significantly more work.

The main obstacle is that if we drop you in the middle of a street, we're just going to automatically assume that you've audited all the way along that street up to the point that you've jumped to. Though we've added an incomplete flag recently that we could probably automatically apply in situations like this. We just need to check downstream to make sure that we aren't adding a bunch of audited distance for users that shouldn't be there, etc.