Consider making the Base link styles into a mixin

[chris-pearce]

It's a common UI pattern to have a <button> look like the default text link e.g. a Button Link component.

Making such a component means you have to copy the CSS for the default text link: https://github.com/campaignmonitor/shell/blob/master/src/_base.scss#L46-L58 which obviously isn't DRY.

Converting the default text link to a mixin will make things DRY as you can then just do this in your Button Link component:

.c-button-link {
    @include base-link;
}

Meaning there's only one source of truth.

[chris-pearce]

Fixed in #92.