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/process/changepassword/AsyncChangePassword.java

@@ -27,7 +27,8 @@ public class AsyncChangePassword implements AsynchronousProcess {
     @Inject
     private PlayerCache playerCache;
 
-    AsyncChangePassword() { }
+    AsyncChangePassword() {
+    }
 
 
     public void changePassword(final Player player, String oldPassword, String newPassword) {

src/main/java/fr/xephi/authme/process/login/ProcessSyncPlayerLogin.java

@@ -14,6 +14,7 @@ import fr.xephi.authme.service.BungeeService;
 import fr.xephi.authme.settings.properties.RegistrationSettings;
 import fr.xephi.authme.service.BukkitService;
 import fr.xephi.authme.service.TeleportationService;
+import fr.xephi.authme.util.StringUtils;
 import org.bukkit.Bukkit;
 import org.bukkit.entity.Player;
 import org.bukkit.plugin.PluginManager;
@@ -90,16 +91,13 @@ public class ProcessSyncPlayerLogin implements SynchronousProcess {
         teleportationService.teleportOnLogin(player, auth, limbo);
 
         // We can now display the join message (if delayed)
-        String jm = PlayerListener.joinMessage.get(name);
-        if (jm != null) {
-            if (!jm.isEmpty()) {
-                for (Player p : bukkitService.getOnlinePlayers()) {
-                    if (p.isOnline()) {
-                        p.sendMessage(jm);
-                    }
+        String joinMessage = PlayerListener.joinMessage.remove(name);
+        if (!StringUtils.isEmpty(joinMessage)) {
+            for (Player p : bukkitService.getOnlinePlayers()) {
+                if (p.isOnline()) {
+                    p.sendMessage(joinMessage);
                 }
             }
-            PlayerListener.joinMessage.remove(name);
         }
 
         if (service.getProperty(RegistrationSettings.APPLY_BLIND_EFFECT)) {

src/main/java/fr/xephi/authme/process/register/ProcessSyncEmailRegister.java

@@ -21,8 +21,8 @@ public class ProcessSyncEmailRegister implements SynchronousProcess {
     @Inject
     private LimboPlayerTaskManager limboPlayerTaskManager;
 
-    ProcessSyncEmailRegister() { }
-
+    ProcessSyncEmailRegister() {
+    }
 
     public void processEmailRegister(Player player) {
         final String name = player.getName().toLowerCase();