Partially addresses #991

This pull request seeks to add a Page Attributes panel to the sidebar for post types supporting the page-attributes support type.

Testing instructions:

Verify that a Page Attributes panel is shown for post types supporting page-attributes, and not shown for other post types. Ensure that only valid input (numbers) is accepted, and that the value is persisted when saving the post.

1. Navigate to Pages > All Pages
2. Hover an existing Page and click Gutenberg
3. In sidebar, note Page Attributes panel
4. Expand Page Attributes and enter numeric order
5. Press Save
6. Refresh page
7. Note order persisted
8. Repeat steps 1 - 3 for Posts, noting that Page Attributes is not present

Ensure that unit tests pass:

npm run test-unit editor/sidebar/page-attributes/test/index.js