New version of DEP 1

This is a more or less complete rewrite of DEP 1. I started with PEP 1, and
made as few changes as possible to modify it to fit our workflow. The
changes from PEP 1 are summarized at the bottom of the new DEP 1.

Thanks to Carl, Jannis, and Andrew for their input on this.

This PR also creates the new directory structure for DEPs, adds a DEP
template, and updates DEP 181 accordingly.
I'd remove "Advisory" here and in rejected (haven't seen that terminology elsewhere)