auth: s/OAuth2Token/BearerToken/g

This also makes some tweaks to the organization of the crypto module to
make it easier for us to add more keys in the future, which will be
necessary for the internal token redesign.

crypto: harden API surface

Make it explicit that expiration is not being tested with Decrypt, and
test that the expiration is meaningful with DecryptWithExpiration.

Add legacy webhooks worker implementation

Overhaul crypto module

Part 1 of merging gql.sr.ht with core-go

Add crypto module with depessimised webhook signing from gitsrht-update-hook