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