Deprecate use of alternateconf.py for debug deployments

[shaib]

alternateconf.py can be used to change definitions for debug deployments. It is currently used also to change the SITEURL setting.

But a better way to change SITEURL -- one that would not require the change of dependent settings (e.g. currently we have FEED_DOMAIN = SITEURL, which means if we override SITEURL in alternateconf.py, we also need to repeat that line there) -- is to set SITEURL from an environment variable. This would make easy to prepare alternate deployments locally, and even easier to make repository forks deploy to alternate sites, because environment variables can be defined at the repo (rather than source) level.

Option added in #22, in aa18c8c