Gutenberg block for setting byline

[mboynes]

The byline meta box on posts should be upgraded to a block for post types using Gutenberg. This should be done when Gutenberg can reliably support a plugin affixing a block to the top of the content area. See attached mockups for what the UI/UX might be like:

Closed state with two byline profiles selected:

Block is active:

Byline override option selected:

[jomurgel]

Update: this still applies, but should be added as a slotfill rather than as a block.

[srtfisher]

Related #74