{WIP} Add pagination component

[emplums]

Fixes: #421
Adds pagination component into primer 🎉

/cc @primer/ds-core

[emplums]

@primer/ds-core not sure if we should include updating the text to use $text-size-small (1px smaller) and the primer spacing system here... it makes quite a different in the overall look of the component and feels a bit off 🤔

em/a/span padding: 7px 12px -> 8px 16px ($spacer-2, $spacer-3)
em/a/span font-size: 12px -> 13px ($font-size-small)
paginate-container margin-top: 20px -> 24px ($spacer-4)
paginate-container margin-bottom: 15px -> 16px ($spacer-3)

Before:

After:

[emplums]

Also - I merged dev into this branch so I could get storybook running - is that ok or should I revert that commit?

[whytrall]

Hey! Check out my comment at #421 (comment).

Again, I'm not sure if it's a good idea to add a new module instead of modifying button group and adding button-group-pagination.

[emplums]

Closing in favor of #487... messed up my commit history a bit!