Disable 'Insert gallery' and 'Set featured image' if nothing is queued

[nacin]

1. If you remove all images from a gallery, the "Insert gallery" button should be grayed out, forcing you to add more images or cancel it. Otherwise you end up with [gallery ids=""].

2. You can click "Set featured image" without having selected a featured image. Again, the button should be grayed out. It closes the modal and fires an unnecessary but not damaging ajax request to delete the featured image. (The XHR returns as unsuccessful as there was nothing to delete.)

[koopersmith]

Patch fixes all button-state issues.

It also:

• Updates the toolbar view to use the view manager (it was the last major holdout — this means that views in toolbars get all the benefits of view management now).
• Updates the default option set to make the API for customizing a single-button flow *much* nicer.
• Updates and comments the custom-background and custom-header flows.

[helenyhou]

All works well for me.

[ryan]

Looks good. Tested gallery, featured image, custom header, custom background, and some free form.

[markjaquith]

All good in my testing.

[nacin]

In 23024:

Media: Disable 'Insert gallery' and 'Set featured image' if nothing is queued. Converts selection toolbars to use the newer view manager. props koopersmith, fixes #22714.

[koopersmith]

Update toolbars for Embed screen. Fixes JS error.

[ocean90]

Looks good. Uncaught TypeError: Cannot read property 'length' of undefined media-views.js:2281 is gone.

[markjaquith]

In 23028:

Update toolbars for Embed screen. Fixes JS error. props koopersmith. fixes #22714