[CHORE] Use github.com/ProtonMail/go-crypto

- We were previously using `github.com/keybase/go-crypto`, because the
package for openpgp by Go itself is deprecated and no longer
maintained. This library provided a maintained version of the openpgp
package. However, it hasn't seen any activity for the last five years,
 and I would therefore consider this also unmaintained.
- This patch switches the package to `github.com/ProtonMail/go-crypto`
which provides a maintained version of the openpgp package and was
already being used in the tests.
- Adds unit tests, I've carefully checked the callstacks to ensure the
OpenPGP-related code was covered under either a unit test or integration
tests to avoid regression, as this can easily turn into security
vulnerabilities if a regression happens here.
- Small behavior update, revocations are now checked correctly instead
of checking if they merely exist and the expiry time of a subkey is used
if one is provided (this is just cosmetic and doesn't impact security).
- One more dependency eliminated :D

routers/web/user/home.go

@@ -35,8 +35,8 @@ import (
 	issue_service "code.gitea.io/gitea/services/issue"
 	pull_service "code.gitea.io/gitea/services/pull"
 
-	"github.com/keybase/go-crypto/openpgp"
-	"github.com/keybase/go-crypto/openpgp/armor"
+	"github.com/ProtonMail/go-crypto/openpgp"
+	"github.com/ProtonMail/go-crypto/openpgp/armor"
 	"xorm.io/builder"
 )