Moves the sidebar padding into the moz-buttons.
This results in an accessibility improvement by giving users a larger
click surface for these buttons.
These changes prevent visual regressions in the sidebar by ensuring
visual consistency in the following cases:
- Pinned tabs in the expanded and collapsed states when using vertical
tabs
- Pinned tabs in the expanded and collapsed states when using
horizontal tabs
- New tab button, actual tabs, and action lists in the expanded and
collapsed states when using vertical tabs
Differential Revision: https://phabricator.services.mozilla.com/D219064
2a57ef1dd6