Enable @mentions in posts and comments

Allow admin, editor and author users to @mention other blog users in posts and comments. The implementation should also include email and/or admin-bar notifications.

The majority of the front-side code is already there. This is already supported as well for o2 on WordPress.com. It is the self-hosted server side work that needs fleshing out.

Start by tracing the existing front-side @mention handling and compare it with the WordPress.com o2 implementation. Then identify the self-hosted server-side entry points for posts and comments and the email or admin-bar notification paths; done means the listed user roles can mention blog users and notifications work.

Written by the indexing model from the issue text.