First pass at styling interaction area buttons

This PR updates the styling of (just) the interaction area buttons. I don't think this is as good as we can do, but it'll help me to see how this version looks on the physical wallscreen. In particular, I'm struggling to judge the best font sizes to keep everything in the right proportion.

I do think the button labels might be a bit too strong now, but we don't have a font-weight option between 400 (the before version) and 600 (the After version). The 400 just feels a bit anemic to me. I'd use 500 but I don't think it exists for Source Sans Pro?

Also, there's probably quite a bit of button-related SCSS refactoring we could do given the more specific styling done here, but I worry about trying to do that before finalizing everything. So maybe after all button styling is done, one of us can come back and remove unused button selectors, etc.

Before

After

sul-dlss / wallscreens

First pass at styling interaction area buttons #230

Before

After