ci: Run yarn before running Storybook

[ActuallyACat]

About

We've had a query about storybook not working locally, which is a result of developers not running yarn install after pulling a branch. This PR adds yarn install to the storybook script.

I was initially worried this would add extra time to run storybook, but in real terms it only adds an additional ~1-2s to the script (assuming the branch is up to date):

$ time yarn
yarn install v1.22.4
[1/5] 🔍  Validating package.json...
[2/5] 🔍  Resolving packages...
success Already up-to-date.
✨  Done in 0.97s.

real    0m1.282s
user    0m1.589s

In my first attempt I wrote a bash script that runs yarn check --integrity, but that will be removed in the near future, and takes a little bit of time.

[mbylstra]

Nice idea!