Twenty Nineteen: add credits for icons

[sabernhardt]

• Adds GPLv2 and Apache 2.0 licenses
• Includes information in the Copyright section about the Material Design icons by Google
• Adds credits to Resources section

Trac 45795

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

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The following contributors have not linked their GitHub and WordPress.org accounts: @khawajaaj.

Contributors, please read how to link your accounts to ensure your work is properly credited in WordPress releases.

Core Committers: Use this line as a base for the props when committing in SVN:

Props sabernhardt.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

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