Update UX of pressing tab bar map button when already active

Many other apps (Twitter, FB, etc.) have established a UX norm where continually tapping tab bar buttons continues to bring you back toward the default state of that tab bar's view controller (e.g., Twitter takes you from tweet detail view to your timeline, then from your timeline to the top of your timeline).

This PR brings this behavior to the OBA app by re-centering the map on current location if the user is already on the map tab. I think it's a nice improvement from currently having to reach thumb from bottom left to top left to achieve same result.

I'm not much of an iOS developer, so I'm unsure of how to write tests for this. Could anybody assist in that regard, or is this simple enough to not require tests?