Fixes partially webhook-ui/1

Background

This PR adapts snapshot-webhook to handle automated webhook subscriptions implemented in: webhook-ui

Changes proposed in this pull request:

• add new api endpoints for:
  • fetching all subscirptions of an owner (:warning: needs further update to secure the endpoint and return the subs only to verified owner)
  • creating and updating a subscription record
• new subscribers module with methods querying DB
• renaming of functions to make a difference between subscriptions from snapshot.org and webhook subscriptions

Remainings TODOS (cross-repo)

• handle verification for fetching all subscriptions from snapshot-webhook, mentioned in the comment in the code
• might be messy, but would be great to rename tables from subscriptions and subscribers to discord_subscribers and webhook_subscribers or something like that 🤔