Mattermost-server: Show more user details in share playbook auto-complete

[mattermod]

Today, the autocomplete for playbook membership shows just usernames:

Let’s update this to show profile pictures, and the full name or username according to the current user’s display configuration:

There is probably an opportunity to unify with the component in the RHS that already does this, modulo any necessary tweaks to preserve location-specific functionality.

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JIRA: https://mattermost.atlassian.net/browse/MM-29288

[BenCookie95]

I can take this one!