Add support for configuring webhooks with commands

[tulir]

Instead of requiring the user manually create the webhook, the bot should have an option to do it automatically. This will also make it possible to fix the webhook secret being public.

• Add command to create, configure and delete room notification webhooks
• Store per-room+webhook secrets in the db and check against those instead of the global secret

[nanonyme]

Please consider prioritizing this. Also access control so that only moderators and admins can manage the webhooks to windows of abuse.

[tulir]

!gitlab webhook add is now a thing which creates a webhook with a random secret that only works for that specific room. Split out permissions and deleting into #46 and #47