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/output/Log4JFilter.java

@@ -12,14 +12,9 @@ import org.apache.logging.log4j.message.Message;
  *
  * @author Xephi59
  */
-@SuppressWarnings("serial")
 public class Log4JFilter extends AbstractFilter {
 
-    /**
-     * Constructor.
-     */
-    public Log4JFilter() {
-    }
+    private static final long serialVersionUID = -5594073755007974254L;
 
     /**
      * Validates a Message instance and returns the {@link Result} value