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package for openpgp by Go itself is deprecated and no longer
maintained.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

tests/integration/user_test.go

@@ -166,8 +166,7 @@ Note: This user hasn't uploaded any GPG keys.
 
 
 =twTO
------END PGP PUBLIC KEY BLOCK-----
-`)
+-----END PGP PUBLIC KEY BLOCK-----`)
 	// Import key
 	// User1 <user1@example.com>
 	session := loginUser(t, "user1")
@@ -201,8 +200,7 @@ C0TLXKur6NVYQMn01iyL+FZzRpEWNuYF3f9QeeLJ/+l2DafESNhNTy17+RPmacK6
 7XhJ1v6JYuh8kaYaEz8OpZDeh7f6Ho6PzJrsy/TKTKhGgZNINj1iaPFyOkQgKR5M
 GrE0MHOxUbc9tbtyk0F1SuzREUBH
 =DDXw
------END PGP PUBLIC KEY BLOCK-----
-`)
+-----END PGP PUBLIC KEY BLOCK-----`)
 	// Export new key
 	testExportUserGPGKeys(t, "user1", `-----BEGIN PGP PUBLIC KEY BLOCK-----
 
@@ -233,8 +231,7 @@ C0TLXKur6NVYQMn01iyL+FZzRpEWNuYF3f9QeeLJ/+l2DafESNhNTy17+RPmacK6
 7XhJ1v6JYuh8kaYaEz8OpZDeh7f6Ho6PzJrsy/TKTKhGgZNINj1iaPFyOkQgKR5M
 GrE0MHOxUbc9tbtyk0F1SuzREUBH
 =WFf5
------END PGP PUBLIC KEY BLOCK-----
-`)
+-----END PGP PUBLIC KEY BLOCK-----`)
 }
 
 func testExportUserGPGKeys(t *testing.T, user, expected string) {