Is your feature request related to a problem? Please describe.

Implement x-posting in WordPress mobile app for sites that have the O2 plugin installed. The + character should trigger a dropdown allowing selection of a site from the list of available sites. This doesn't include the UI to add a x-post via the toolbar menu (which is covered by #2612).

Describe the solution you'd like

Gutenberg on Calypso uses the https://example.com?get-xpost-data endpoint, which is provided by the O2 plugin.

Describe alternatives you've considered

The /me/sites endpoint on the WordPress.com API returns the user's sites, and is already implemented by the WordPress mobile apps. The downside is that it doesn't perform the additional filtering that the O2 plugin does, so if we were to implement it, we'd have to duplicate logic, e.g.:

• Filter out the current site
• Filter out sites that have incompatible privacy settings
• Any other logic

Leveraging the ?get-xpost-data allows us to avoid duplicating any of this logic.