Iconography: Audit icon usage

[sixhours]

A couple tasks here:

• In the short term, remove Material icons from the sidebar; we can replace these with Gridicons for now. This brings more consistency to Calypso (see Icons: List view ellipsis menus are using old Gridicons. #37116)
• Audit Gridicons against WP core's icon set to see what might be missing for WordPress.com's purposes. Eventually it would be ideal to be able to use the same set of icons throughout both systems for consistency.

See pcbO8q-bb-p2#comment-183 for backstory.

[github-actions]

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