Release Talkyard v1

Currently preparing Talkyard v1 — a major new version (a new epoch).

Installations instructions: https://github.com/debiki/talkyard-prod-one/tree/ty-prod-one-v1

Upgrade instructions: https://github.com/debiki/talkyard-prod-one/blob/ty-prod-one-v1/docs/upgrade-v0-to-v1.md
To upgrade from Talkyard v0 to v1, you install Talkyard v1 side-by-side with v0, backup v0, and restore the backup to v1.

Talkyard v1 upgrades 3rd party software, e.g. Postgres, ElasticSearch. Talkyard v0 will stop getting new features, only bug fixes.

The most interesting changes are, I think:

• Backups in /var/opt/backups/talkyard/v1 (instead of /opt/talkyard-backups/. Now trying to follow the Filesystem Hierarchy Standard).
• Docker named volumes (instead of bind mounted "data directories" in /opt/talkyard...).
• Docker secrets in /opt/talkyard-v1/secrets/ (the Postgres password and an optional backup encryption password).
• A backup container that runs the backup scripts (instead of running backup scripts directly on the host).
• Using the Docker local logging driver (for automatic log rotation), instead of writing to /var/logs/....
• Security, e.g. dropping capabilities (cap_drop: ALL, cap_add: <what's needed>), upgrading everything.