add FancyboxButton Storybook stories

[laespinoza]

Adding this issue that came out from this pull request discussion:

• add a story for the fancybox button and a corresponding fancybox link

[stephiescastle]

I'll work on adding this as part of #105

[stephiescastle]

I decided to tackle this separately from #105. I created a branch that adds a story, but I'm thinking of pushing this off for a later release.

The FancyboxButton template we have and are currently using throughout our templates makes a lot of assumptions under the hood, and trying to document how to use it alongside Fancyapps' own documentation is confusing.

Essentially, users just need to know what data attributes to add to a an HTML element. The names of the data-attributes don't line up with our prop names, and I'm not sure it's worthwhile to rename/restructure everything for this reason. Unfortunately, fancyapps doesn't have great documentation about all of the data attributes that are available for fancybox, so we may want to consider adding our own docs on that, but I don't think it's necessary for the 1.0.0 release.