Fix various code issues as detected by Sonar

Mostly minor changes:
- Add deprecated javadoc tag on deprecated members
- Reduce duplication (FlatFile, BackupService, ...)
- Make methods static
- Reduce size of anonymous classes
- Replace name with displayName in PermissionsSystemType (avoids confusing with Enum name())
- Tabs to spaces
- Merge if statements

Code from third-party sources (BCryptService, BinTools, PHPBB) not modified.

src/main/java/fr/xephi/authme/permission/AuthGroupHandler.java

@@ -31,7 +31,8 @@ public class AuthGroupHandler implements Reloadable {
     private String unregisteredGroup;
     private String registeredGroup;
 
-    AuthGroupHandler() { }
+    AuthGroupHandler() {
+    }
 
     /**
      * Set the group of a player, by its AuthMe group type.