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/service/ValidationService.java

@@ -40,7 +40,8 @@ public class ValidationService implements Reloadable {
     private Pattern passwordRegex;
     private Set<String> unrestrictedNames;
 
-    ValidationService() { }
+    ValidationService() {
+    }
 
     @PostConstruct
     @Override