Add Review and Follow Up to PR Template

[teor2345]

Motivation

We've been talking about our PR process for the last few weeks, and we've identified some issues:

• we don't know who should review a PR
• we don't know how urgent PR reviews are
• it's hard to tell how much of a design or ticket is implemented in a PR
• we don't consistently track follow-up tasks
• sometimes we forget documentation or tests

Solution

Make changes to the PR template:

• Add a Review section
• Add a Follow Up section
• Add a checklist to Solution with docs and tests
• Edit some existing section text so it's shorter

Related Issues

#1177 was my original suggestion, but it's way too long.

Follow Up

Move the reviewer guidelines from #1177 into the dev guide in the book, if we think that's a good idea.