Extracted private methods from import users controller to a lib module a...

...nd added comments. Changed user model to only skip phone validation when importing from csv.

Fully resolves #212. The validations work, and the input users controller is now small enough to write tests for. 

A good next step might be to extract the validations and associated messages from the import users controller to a custom validation class. That said, it's unlikely we'll need to write any more and there are only four so we could just leave it for now.