Add Percy tests from storybooks

[felixfbecker]

Percy supports screenshotting storybooks.

• This way we can expand our stoybook library and get free unit tests for free
• It adds a layer of testing we are severely undertesting, which is styling
• It makes it easier for the design team to review changes, as they can review screenshots in Percy
• One screenshot can replace dozens of assertions, and is way easier to evolve

First came up in https://sourcegraph.slack.com/archives/CMT39K56Z/p1579792526049200

[uwedeportivo]

Dear all,

This is your release captain speaking. 🚂🚂🚂

Branch cut for the 3.16 release is scheduled for tomorrow.

Is this issue / PR going to make it in time? Please change the milestone accordingly.
When in doubt, reach out!

Thank you