Add Telegram integration

[beeman]

Currently, the system is built around Discord role validation but it's been designed to add support for other platforms relatively easily, and the single most requested platform is Telegram.

• feat: prepare the platform for multiple bots #94
• Add a way for users to verify their Telegram identity and link it to their account. Telegram Link & Login #97
• Add support for a Telegram bot configuration (per community).

This is the main issue but the work will be split out into (at least) 3 separate PRs.