mod_http_oauth2: Enforce client scope restrictions in authorization When registering a client, a scope field can be included as a promise to only ever use those. Here we enforce that promise, if given, ensuring a client can't request or be granted a scope it didn't provide in its registration. While currently there is no restrictions at registration time, this could be changed in the future in various ways.

Over the years, some servers stop working for various reasons, and leave
behind broken roster subscriptions that trigger failing s2s connections.
This module allows cleaning up such cases by unsubscribing local users
from their contacts on those servers. Also works for typos and the like.
Use with care.

```lua
Component "gmail.com" "unsubscriber"
```