CSS units refactor

• Refactored CSS units to use rems instead of ems based on html root font size
• Adjusted sizes slightly in some places to be more consistent with pre-rem version
• Set height and width on profile image to prevent flicker on transition between pages
• Set max-width on profile header and bio to match width of <main>
• Replaced <span> that wraps posts with a more semantic (block level) <article>—we should do more work on the semantics of the elements in this area

While the refactor did allow me to remove the overrides previously in place to make Bulma components use ems, I found that Bulma is using ems in the padding for tabs. So I still have to override Bulma for that. But it’s only one rule, rather than lots of rules!