Create <empty-state-webdash> component for webdash plugins

[jadjoubran]

Promotes consistent empty state design

Once the element is built, it needs to be refactored in:

• [x] publish component on webcomponents.org
• [x] webdash-package-json
• [x] webdash-performance-budget
• [x] webdash-npm-scripts
• [x] webadsh-readme-preview
• [x] webdash-pwa-manifest
• [x] generator-webdash-plugin
• [x] new plugins that might have emerged
• [x] update documentation
• [x] link empty-state-webdash's readme to webdash documentation

[jadjoubran]

🎉