Consolidate mastodon and webserver-content roles

Summary:
The mastodon role has only a webserver content unit,
so it could be moved to the webserver-content role.

This provisions https://social.nasqueron.org/support.

This is a follow-up for 92170abb2.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1264