refactor: use material icons with Newspack color

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

A small refactor which replaces hard-coded Material Design SVGs with icons from the Material Design Icons NPM package. Also ensures that all icons are displayed with the Newspack brand color where appropriate. Shouldn't affect any plugin behavior.

Also updates some docblock comments to maintain consistency between the use of "WordPress" vs. "External" dependencies.

How to test the changes in this Pull Request:

1. Check out this branch, run npm ci && npm run build.
2. On a post or page, add a Curated List block. Confirm that the block icon is now blue to match other Newspack icons:

3. Select "Specific Listings" mode. Add blocks for each listing type to the list (you'll need at least one published listing of each type to add them to the list block). Confirm that the individual icons for each different type are now Newspack Blue in both the block inserters and the placeholders:

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[adekbadek]

LGTM

[matticbot]

🎉 This PR is included in version 2.0.0-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀