Create builder for command description; refactor command classes

- Create builder class for CommandDescription
- Remove redundant methods in command registration classes (e.g. certain validation is superfluous because we only instantiate the classes internally)
- Replace multiple occurrences where a field is directly instantiated e.g. with a list and then its value is overwritten by a constructor = redundant instantiation of objects

src/main/java/fr/xephi/authme/command/help/HelpPrinter.java

@@ -44,7 +44,7 @@ public class HelpPrinter {
             sender.sendMessage(ChatColor.GOLD + "Short Description: " + ChatColor.WHITE + command.getDescription());
 
         // Print the detailed description, if available
-        if (command.hasDetailedDescription()) {
+        if (!StringUtils.isEmpty(command.getDetailedDescription())) {
             sender.sendMessage(ChatColor.GOLD + "Detailed Description:");
             sender.sendMessage(ChatColor.WHITE + " " + command.getDetailedDescription());
         }