Fix responsive issues & a dimming issue

[jasmussen]

This PR fixes an issue with the dimming layer on the cover image, and then it does a fair bit of rewriting of the sidebar code to make it work better on mobile.

It's still a bit of a work in progress, but I'm assigning reviewers early because the CSS is sort of gnarly and I'd appreciate insights.

Also — while working on this, I discovered that WordPress has already solved CSS bleed with a class called modal-open. It's pretty magical, and I would love it if we can apply this to the sidebar when on mobile. See more info here: #656 (comment) — but this should probably be done in a separate PR.

Steps to test:

• Verify that the dimming layer on cover image looks right on all alignments
• Verify that the sidebar works / looks well on desktop, including with all metaboxes open
• Verify that the sidebar works well as a modal on mobile, including with all metaboxes open

[jasmussen]

Actually this isn't in progress. I'm just expecting code review comments.

[aduth]

Just to clarify, by "modal", do you mean it should be expected that the sidebar covers the entire screen on mobile, including the admin bar? (This is what I am seeing)

[jasmussen]

Just to clarify, by "modal", do you mean it should be expected that the sidebar covers the entire screen on mobile, including the admin bar? (This is what I am seeing)

Yes. Having tried various things, this is what seems to work the best for me. It fixes both an issue with scroll bleed, as well as an issue with sticky toolbars that may or may not be scrolled out of sight.

Thanks for the review!