Add loading spinner

[jgzuke]

Designs at https://app.zeplin.io/project/5c7dcb5ab4e654bca8cde54d/screen/5d56d40acf2df541b112fc57

Spinner code mostly from https://codepen.io/daniman-the-bold/pen/jgjqBw, thanks @daniman 🎉

Also made the DemoGroup shared since it was being used in TextField, Button, and now Loader stories.

[justinanastos]

Left some questions and suggestions. Reach out to me with any questions!

[justinanastos]

This also needs some input from @adamatorres about how to handle the disabled button state where we can't see the orbit

[daniman]

This is pretty great Jason, thanks for putting my pen into code here! Two quick comments on the styles:

#1 –– I think the orbit line and orbit circle probably need a bit of tweaking at the different sizes. I'm not sure what the dimension are myself.

#2 –– I think Tim had a different timing function for the orbit when it's in a button. Looks like it's a slightly faster linear animation, not a cubic bezier. You should sync up w him and Adam on the intent there: https://meteor.slack.com/archives/CFSCN3G7L/p1565744964008200.

[justinanastos]

Thanks for making those changes! I'm leaving a comment on a resolved comment from before so I want to make sure you see this:

I didn't say this in the comment so there's no fault on your part; I was hoping you'd respond to this message instead of just making the change and requesting a new review.

It's important to me that you understand why I'm requesting this change and why I think this makes future code changes safer. Why? If you are looking for the same things, then you'll catch places where I might have missed strong type safety in my PRs. A rising tide lifts all ships!

Never be afraid of communicating too much!

[justinanastos]

Get eslint to pass (the error is auto-fixable, looks like it's time to add a global vs code extension that auto fixes things).

[justinanastos]

Get eslint to pass and merge away!

[justinanastos]

🚀 PR was released in v1.8.0 🚀