Suggest a rebase/squash before pull requests

Performing a rebase/squash before a pull request performs an opportunity to
craft well-written commits before reviews are performed. This makes the review
easier, as the commits are easier to understand, and it allows review of the
actual commit titles and messages.
Remove guide for using single mailer

This is not a guideline we often follow. In addition, it's fairly
restrictive and leads to poor organization of information that could be
better expressed with proper naming. If anything this guide violates our
other guide for naming things to reveal intent.