pki: improve serial number generation + fix team association

Replace math/rand by crypto/rand.

Fix big when associating certificate with leading zero: nginx prepend 0 wherehas we don't.

libfic/db.go

@@ -84,7 +84,7 @@ CREATE TABLE IF NOT EXISTS teams(
 	}
 	if _, err := db.Exec(`
 CREATE TABLE IF NOT EXISTS certificates(
-  id_cert BIGINT NOT NULL PRIMARY KEY AUTO_INCREMENT,
+  id_cert BIGINT UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
   creation TIMESTAMP NOT NULL,
   password VARCHAR(255) NOT NULL,
   id_team INTEGER NULL,