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/data/auth/PlayerAuth.java

@@ -189,7 +189,7 @@ public class PlayerAuth {
      * @return String
      */
     public String serialize() {
-        StringBuffer str = new StringBuffer();
+        StringBuilder str = new StringBuilder();
         char d = ';';
         str.append(this.nickname).append(d);
         str.append(this.realName).append(d);