Auto-populate pull request "body" with commit message (BB-11936)

It would be extremely continent if a pull request's "body" was auto-populated from the "body" of a commit.

For example, the commit message:

foo: Fix the broken thing

Some descriptive stuff about how the thing was broken, and what was done to fix it. This is separated by the first ("header") line by two newlines.

Should yield a PR with:

title: "foo: Fix the broken thing"

body: "Some descriptive stuff about how the thing was broken, and what was done to fix it. This is separated by the first ("header") line by two newlines."

GitHub does this and it saves me from having to copy/paste my commit messages over and over again.