Fix for #1875 | buttons and tags group sizing (are-small/are-medium/are-large)

[diomed]

This is a new feature.
Proposed solution

This patch is a new feature to accommodate the request in #1875. The issue has a request for sizing classes on the the .buttons and .tags elements. This PR adds that feature by adding are-medium and are-large classes to the tag.sass file and are-small, are-medium, and are-large classes to the button.sass file.

Tradeoffs

Using the sizing classes on the .tags or .buttons elements removes the ability to size the children individually. For instance, if you used .are-medium on the parent element and then tried to give one of the children .is-small, it wouldn't apply the .is-small styling.

If the user wishes to have differently sized buttons, they will have to add the desired sizing class to each .button element. This would be no different than how they currently have to size buttons differently.

Testing Done

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hijacked pull #1959 from @iamtimsmith (Thank You) - which can be closed now