Modify `new_site` to accept a SITE_TYPE parameter

When `rake new_site[abbr,slug,host]` is called, `SITE_TYPE=redirector` or `SITE_TYPE=bouncer` 
can be specified. If neither is given, `bouncer` is the default and a site will be created in `transition-sites`.

Using `export SITE_TYPE=redirector` can change the default if this is not desirable in the short term.