Merge CommandService and ProcessService to CommonService

- Replace CommandService and ProcessService with CommonService: a service that offers our typical needs to work with settings, messages and permissions
- Remove validation methods from CommonService: inject ValidationService directly. Validation methods are not used very frequently and therefore don't belong in CommonService. Their presence was a relict from our architecture before injection was used.

src/main/java/fr/xephi/authme/process/unregister/AsynchronousUnregister.java

@@ -9,7 +9,7 @@ import fr.xephi.authme.message.MessageKey;
 import fr.xephi.authme.permission.AuthGroupHandler;
 import fr.xephi.authme.permission.AuthGroupType;
 import fr.xephi.authme.process.AsynchronousProcess;
-import fr.xephi.authme.process.ProcessService;
+import fr.xephi.authme.service.CommonService;
 import fr.xephi.authme.security.PasswordSecurity;
 import fr.xephi.authme.settings.properties.RegistrationSettings;
 import fr.xephi.authme.settings.properties.RestrictionSettings;
@@ -31,7 +31,7 @@ public class AsynchronousUnregister implements AsynchronousProcess {
     private DataSource dataSource;
 
     @Inject
-    private ProcessService service;
+    private CommonService service;
 
     @Inject
     private PasswordSecurity passwordSecurity;