Anchor: Allow users to connect an existing account as their podcasting site.

[kwight]

Anchor users currently have no choice but to create a new WP.com site when they want to use our integration. Both sides have gotten user feedback requesting the ability to use existing accounts for the imports.

The primary concern with using existing accounts would be making sure the user can differentiate their imported content (and that nothing is overwritten). Inserting the posts with a podcast-episode tag or something is one option. If we want to use a category though, we'd first need to add support for categories in Headstart, which was a challenge in the past (they're hierarchical taxonomies, while tags are not, and that caused issues with Headstart's lifecycle requirements).

See: 500-gh-Automattic/dotcom-manage