Summary:
- Hides left and right panels and the top bar
- Hides cursor and active-section highlight
- Hides "=" icon on formulas
- Nudges browser to include background for row/column headers, which is not
otherwise included.
Still only what's visible is printed (e.g. large tables not paginated).
Test Plan: Tried a few pages manually on Firefox and Chrome.
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D2579
