Phase out material UI as a dependency

[bintzandt]

Summary

We are currently using material-UI as a dependency in our projects. We want to stop using material UI as a dependency in the long run.

Explanation

• [ ] Remove material-ui dependency from packages/yoast-components/composites/OnboardingWizard/OnboardingWizard.js
• [ ] Remove material-ui dependency from packages/yoast-components/composites/OnboardingWizard/LoadingIndicator.js
• [ ] Remove material-ui dependency from packages/yoast-components/composites/OnboardingWizard/StepIndicator.js
• [ ] Remove material-ui dependencies from packages/yoast-components/composites/OnboardingWizard/tests/WizardTest.js
• [ ] Remove material-ui dependencies from packages/yoast-components/composites/OnboardingWizard/config/yoast-theme.js
• [ ] Remove material-ui dependencies from packages/yoast-components/composites/OnboardingWizard/StepButton.js
• [ ] Remove material-ui from the packages/yoastseo/examples/webpack/package.json
• [ ] Remove material-ui from the packages/yoast-components/package.json
• [ ] Remove material-ui from all the comments and README.md files

Technical decisions

Other components should be created to replace the components from material-ui.

[diedexx]

This issue can be split up in multiple, smaller issues. When that is done, this issue can be closed.