Local service instances

Add Ansible playbooks that install major services as VMs:

• Synapse #12 (closed)
• Mastodon #19 (closed)
• Prosody #20 (closed)
• Dovecot #21 (closed)

For all services:

• Packages on "Debian stable" or "Debian stable backports" are preferred, otherwise deb sources close to upstream
• Only LDAP-based login functionality is required
• If it's possible to authenticate two accounts from two clients, and message each other, that would be nice, but not required

Notes:

• Feel free to create a new ticket for each service
• Go for the low-hanging fruit
• Create tickets for any blockers