[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package for openpgp by Go itself is deprecated and no longer
maintained.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

models/asymkey/gpg_key_common.go

@@ -13,9 +13,9 @@ import (
 	"strings"
 	"time"
 
-	"github.com/keybase/go-crypto/openpgp"
-	"github.com/keybase/go-crypto/openpgp/armor"
-	"github.com/keybase/go-crypto/openpgp/packet"
+	"github.com/ProtonMail/go-crypto/openpgp"
+	"github.com/ProtonMail/go-crypto/openpgp/armor"
+	"github.com/ProtonMail/go-crypto/openpgp/packet"
 )
 
 //   __________________  ________   ____  __.
@@ -88,7 +88,7 @@ func getExpiryTime(e *openpgp.Entity) time.Time {
 	for _, ident := range e.Identities {
 		if selfSig == nil {
 			selfSig = ident.SelfSignature
-		} else if ident.SelfSignature.IsPrimaryId != nil && *ident.SelfSignature.IsPrimaryId {
+		} else if ident.SelfSignature != nil && ident.SelfSignature.IsPrimaryId != nil && *ident.SelfSignature.IsPrimaryId {
 			selfSig = ident.SelfSignature
 			break
 		}