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/permission/handlers/BPermissionsHandler.java

@@ -55,7 +55,7 @@ public class BPermissionsHandler implements PermissionHandler {
         List<String> groups = getGroups(player);
 
         // Make sure there is any group available, or return null
-        if (groups.size() == 0)
+        if (groups.isEmpty())
             return null;
 
         // Return the first group

src/main/java/fr/xephi/authme/permission/handlers/PermissionsBukkitHandler.java

@@ -54,7 +54,7 @@ public class PermissionsBukkitHandler implements PermissionHandler {
         List<String> groups = getGroups(player);
 
         // Make sure there is any group available, or return null
-        if (groups.size() == 0)
+        if (groups.isEmpty())
             return null;
 
         // Return the first group

src/main/java/fr/xephi/authme/permission/handlers/PermissionsExHandler.java

@@ -77,7 +77,7 @@ public class PermissionsExHandler implements PermissionHandler {
         PermissionUser user = permissionManager.getUser(player);
 
         List<String> groups = user.getParentIdentifiers(null);
-        if (groups.size() == 0)
+        if (groups.isEmpty())
             return null;
 
         return groups.get(0);