Accessibility: screen reader handling of map zoom level changes

[aviannamiller]

Map zoom controls currently lack status announcements. Screen reader feedback should note the new map zoom level after successful control activation. This goes for zoom in, zoom out, and fit all.

[jimsafley]

This is bigger than the title suggests. The good news is that nothing here is blocked by Leaflet, unlike the issues in #75. The announcements are ours to build, but the screen-reader plumbing only exists on about half our maps, "fit all" needs separate handling from zoom in and out, and we'd have to keep maps from announcing their own automatic movements. It should also be done alongside @aviannamiller's comment on #75, since it's the same plumbing.

Worth doing, but it's a feature-sized project across most of our map views rather than a small fix. We should defer past the current release rather than holding the release up for it.

[aviannamiller]

That sounds like a plan.

I've updated the title to hopefully be more reflective of the issue scope. Feel free to make any changes or additions where you see fit.