Speed up changing speed limit between labels on Validate page

[misaugstad]

Brief description of problem/feature

@jsomeara is adding a speed limit sign to our Explore/Validate UIs in #3611. While testing, I noticed that the Overpass API can occasionally take multiple seconds to respond. We're leaving the old speed limit up until the new one is found. I think that this is fine for the Explore page, where you're generally walking down a single street where the speed limit remains constant.

But on the Validate page, you're constantly switching contexts, so sometimes the speed limit is incorrect for awhile. It's also a little distracting for the speed limit to change after everything else has loaded. We should have the speed limit updating instantly when you switch to a new label!

I'm making a new ticket for this because I think that it's out of the scope of that PR, and we should have people test out the new feature before we continue to put dev resources into perfecting it.

Potential solution(s)

Since we know what the 10 labels that a user is validating in the current mission at the beginning of the mission, we should just get the speed limit for all 10 panos right at the beginning of the mission and store them. Should make it easy to instantly update the speed limit as we move on to new labels in the mission.

Not sure if we can run just one Overpass API query or if we'll still need to do 10 separate ones. And not sure whether it's worth it to check for duplicate pano_ids from the list of labels to occasionally reduce the number of queries we make. If it's worthwhile, we could also store the speed limit not just with the label, but with the pano metadata as well. Which could mean that the longer that a user validates without refreshing the page, the less likely it is that we need to query Overpass for a given label.