Add handling of Discussion webhooks

[sario528]

List of webhook actions to handle for Discussions:

created
edited
deleted
pinned
unpinned
locked
unlocked
transferred
category_changed
answered
unanswered

And Discussion comments:

created
edited
deleted

[dgw]

I wonder what transferred is: moved to another repo, like issues, or converted from an issue. 🤔

[sario528]

I wonder what transferred is: moved to another repo, like issues, or converted from an issue. 🤔

I did some quick testing, and the 'transfer' is between repos.