Deprecate Largo's custom byline feature

[benlk]

From discussion on #1248 for #1126, we should look to deprecate this and replace it with Co-Authors Plus across the board.

[benlk]

To assist people with fixing this, we should release a plugin that:

• shows a list of posts with custom bylines, with edit links
• shows a list of custom bylines, with instructions on how to create CAP guest authors for each
• stretch: does a Term Debt Consolidator thingy to create CAP guest authors
• preserves the Largo custom byline functionality once Largo drops the custom byline code

[benlk]

"custom byline" here means the arbitrary-text, arbitrary-link metabox, not largo_byline(). It's kind-of plugin functionality.

[benlk]

Do we want to deprecate Largo's custom byline feature?