Rename partner.utils to partner.importers, and comments

It only contains importers. Also added comments about their usage,
or lack thereof.
Move demo site importer and commands into Partner app

That way, they aren't left to rot in the bigbang app. Gives us one
central place to refactor them.

Fixes #1238
Remove demo.bigbang app

It's empty now, so can safely be removed.
Universal newline mode for CSVUnicodeReader

Some demo site fixtures have Windows line endings, and this fixes it.
Universal newline mode is the default for Python 3 anyway, so it makes
good sense.