Add a "maxed out zoom animation" to indicate max zoom has been reached.

This issue has been migrated from mozilla/webmaker-android#1797.

It was originally written by flukeout on Mon May 11 2015 and had the following description:

I think on either end of the zoom spectrum we should add a little bounce to indicate that you can't go further. I can provide detailed spec for this but something to this effect:

• Put map in fully zoomed out state
• Attempt to zoom out more
• The map will zoom just a tiny bit more out but will stop after that even if you continue zooming
• When you release your fingers the map will bounce back to the fully zoomed out state

I think these 'rubber boundaries' will help users realize that they're at the max zoom level. Right now there is just a hard stop and I find myself trying to zoom out beyond it just thinking my gestures aren't registering.

**@**vazquez Thoughts?