Adjust static asset handling by Nginx to accommodate Django

This changeset alters the way static assets are served by Nginx. Static
assets emitted by Django and prefixed with `/static/` are handled by a
specific `location` block that creates an alias to
`{{ app_home }}/collected_static`.

For other static assets prefixed by `js` or `img` or `css` or `font`
(taken from the `.gitignore` file inside `src/nyc_trees/static`), Nginx
looks for a match within the web root and returns a 404 if it is not
found. This prevents Nginx from passing static file misses through to
Gunicorn.

Attempts to resolve #78.