pin yarn-1.21.1

Following the recommendations at
https://legacy.yarnpkg.com/en/docs/cli/policies , I ran `yarn policies
set-version 1`, which added a copy of the now-current 1.21.1 to `.yarn/` and
added a `.yarnrc` to use it. I believe no matter what version of Yarn you
have installed locally, when you run `yarn COMMAND`, your yarn will delegate
everything to this yarn-1.21.1 .

Now that yarn-2 has been released, we'll need this pinning to keep things
working even when developers have yarn-2 installed on their machines.

refs #451