Sidebar: fixup styles to work without the CSS reset

[jsnajdr]

While playing with the Gutenlypso prototype in #44801, I noticed a broken sidebar:

It's caused by missing styles that used to be directly on the ul and h2 elements in the CSS reset. This PR adds them to the specific .sidebar_... rules.

How to test:
On master, this change shouldn't have any observable effects. In the #44801 branch:

• the horizontal identation of sidebar items should be removed
• the vertical space between items should be smaller
• after clicking on an expandable item, e.g., "Site", you shouldn't see a blue focus ring (added by default on the <h2 tabindex=0> that has been made interactive by the tabindex attr)

[matticbot]

Test live: https://calypso.live/?branch=fix/resetless-sidebar-styles

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[tyxla]

LGTM, sidebar looks great with #44801 now 👍

🚢

[sarayourfriend]

Thanks for doing this @jsnajdr! It's also part of #45259.