Add map reset button

[tomay]

Add a new button to reset the map, this should go next to the existing RESET ALL button

I suppose a question is whether we need a new button, or if we just extend the existing button functionality to include a map reset (start x, start y, start zoom)

[tomay]

For now, I've added a few lines to reset the map. This also gets triggered when one clicks the "RESET ALL" button.

This could be easily be factored out into a new button. But part of my hesitation is that there's not a lot of room here for a second button, and the words "RESET ALL" seem like they should apply equally to the map.

One note, we reset to map defaults defined in seattle.json not the map extent and zoom that comes from the initial URL params. That seems to me like the most obvious reset, but there may be other opinions on that

[tomay]

Change text to "RESET MAP", instead of "RESET ALL"