Iterate on drawer styling in Boards

Problem to solve

The drawer documentation has been updated a bit since we first started working on it. To make the drawer in Boards look more consistent with the other panels- such as the sidebar - we should apply some of the documented updates to it.

Current drawer state	After

User experience goal

Different panels within GitLab are easily identifiable as similar objects.

Proposal

Drawer iteration
drawersidebar

Drawer updates:

• Figma: https://www.figma.com/file/rUIRiMDthKJsCjrjBS3IOv/244633-Iterate-on-drawer-styling-in-Boards?node-id=1%3A16915
• Width of the drawer should match the sidebar (~290px here)
• Drawer is $gray-50 (which I believe the sidebar will be updated to as well)
• Header is UI/h2/04 base