Add a tooltip that explains what the resync button does

The Backstroke dashboard has a button that manually resyncs links. Because of space reasons, it's not very descriptive. It's be really neat if hovering over the button showed a tooltip that explained more about the button's purpose.

Ideally, it would also be great if this tooltip was a reusable component that could be used in other places on the page too. We have a handy script that will scaffold out a component for you, check it out with `yarn make-component` or [look at the source here](https://github.com/backstrokeapp/dashboard/blob/master/utilities/make-component).

Here's a design mockup of what I had in mind, though feel free to suggest better ideas:

Thanks for contributing!