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/command/executable/email/ShowEmailCommand.java

@@ -23,7 +23,7 @@ public class ShowEmailCommand extends PlayerCommand {
     @Override
     public void runCommand(Player player, List<String> arguments) {
         PlayerAuth auth = playerCache.getAuth(player.getName());
-        if (auth.getEmail() != null && !auth.getEmail().equalsIgnoreCase("your@email.com")) {
+        if (auth.getEmail() != null && !"your@email.com".equalsIgnoreCase(auth.getEmail())) {
             commandService.send(player, MessageKey.EMAIL_SHOW, auth.getEmail());
         } else {
             commandService.send(player, MessageKey.SHOW_NO_EMAIL);