Add link to tracker from all world-specific threads

Add a direct link to the the multiworld tracker to the opening message of each per-world thread in all threaded output channels. Tracker link to world is composed as: <host>/tracker/<instance hash>/0/<world number>.

• Store world number as part of world configuration - save from maintenance module when parsing spoiler log
• Add base URL to watcher config (host up through <instance hash>)
• Derive remaining tracker link from stored world number