DRY up references to the add tree URL hash

We funnel users into adding trees from many places in an effort to maximize user contributions. As a result, construting the url to the add tree mode of the map page was scattered in a few different files. I unified this as best I could by moving the hash text out to ``settings.py`` and then forwarding it through to the templates and the client-side code, via the templated ``settings.js``.

Fixes #635