[Feature] Add Featured Image Control

[jessecowens]

Is your feature request related to a problem? Please describe.
Currently, a blog post's featured image appears as the background of the post title/meta. This severely crops the image and may cause readability issues.

Describe the solution you'd like
A control should be added to Customize -> Design -> Blog -> Posts menu allow the user to select whether the featured image should be displayed as the title background or at the top of the post content.

Describe alternatives you've considered
The featured image is used as the Open Graph image by many SEO plugins, so simply removing the featured image to prevent this display is not ideal. The user can use Custom CSS to choose a solid background color.

Additional context