Make all details of "Add Tree" steps work

* Move most tree-specific parts of addMapFeature.js to addTreeMode.js
* Create stepControls.js to manage step navigation (Note because there's a CSS "transition" animation we need events for both the start and the end of a step change)* Enable/disable first "next" button as you go through the "choose location" process
* Focus on appropriate input element when you change steps
* Get data for final "summary" step (and show "Empty planting site" or "Missing species" as appropriate)
* Set hidden address fields correctly fromm reverse-geocode (fixes #1174)