Migrate Gutenberg theme support options into checkboxes.

[kjellr]

This PR takes the optional Gutenberg theme support declarations that the Gutenberg Starter theme uses, and migrates them into checkbox options on the "Theme Options" settings page created in #85. (Note that this page now lives under the "Appearance" menu). Moving these here will provide a quick way for developers to enable and disable these options while testing Gutenberg.

• align-wide
• editor-color-palette
• wp-block-styles (the "opinionated" styles required to support the core blocks in the front end.)
• responsive-embeds

Because all of these had previously been enabled by default, the new checkboxes default to checked.

Testing

• Install the PR version of this theme on a test site, do not change any settings yet.
• Ensure that the options above are all active by default:
  • You should see wide and full alignment options available in the editor.
  • You should see the custom theme color palette available in the editor.
  • You should see default block styles applied on the frontend. (A quick way to check is to verify that the quote block appears with a left border).
  • On the front end, the body should have a wp-embed-responsive class.
• Check and uncheck the new and verify that they toggle the options as expected.

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Screenshot: