Previously, rb-site would hand-craft a settings_local.py for the

generated site. This was a bit messy to maintain, though, and very

limiting. A request we've had in the past is to allow a custom template

to be passed to rb-site, and we had no way to offer this for this

file.
This change moves us to using a template file, in the same format as the

others we generate (like for crontabs). We build the data up-front, pass

it to the template, and write it to the site directory.
There are some differences. Because we're having to use json to

generate the structured data, we're stuck with some annoyances like a

lack of trailing commas in lists/dictionaries, and double quotes instead

of single quotes. Not a big deal, since this file isn't meant to be

worked with all that much, and these are purely stylistic differences,

but there isn't a better approach without a whole lot of extra work.

Unit tests pass.
Ran rb-site install and verified the resulting file looked correct and

had the proper generated settings.