hr/border refactor - Githubissues

this PR addresses some problems with borders on cards & the modal: they had subtle variations in spacing/color, were tied to specific elements using padding/margin, and sometimes didn't appear if the sequence of elements wasn't right (e.g. no experience subtitle).

the new approach uses the <hr/> element so that borders can be explicitly inserted into the html, and are guaranteed to appear regardless of order/placement of surrounding elements. they are all styled using the same CSS.

relatedly, this PR removes some styles that hewed too closely to figma in favor of unifying margins across experiences (for example, the spacing between experience title and subtitle).

Don't render empty experience subtitles
Use horizontal rules for card/modal borders
Globally unify margins/padding for hr

sul-dlss / wallscreens

hr/border refactor #184