Login with multiple forges

[anbraten]

🚀 This PR allows to select a forge to login with. For now users can only use the forge they logged in with.

Changes

• [x] list all configured forges on Login page
• [x] enhanced webhook parsing by directly checking the secret
• [x] test for webhook endpoint

[woodpecker-bot]

Deployment of preview was successful: https://woodpecker-ci-woodpecker-pr-3822.surge.sh