#792 #814 Create command to remove NOT NULL constraints

- Create command under /authme debug that allows to change the 'nullable' status of MySQL columns (currently last date and email only)
   - We need to offer a default value for forum integrations that have a NOT NULL email column. Offering a command avoids us from force-migrating existing databases while still offering migrations in both directions
- Change in default value handling: lack of values are not handled by setting default values to the PlayerAuth anymore, and reading a default value from the database into a PlayerAuth will be translated into null by the PlayerAuth builder
- When a new database is created, email and lastlogin are now nullable and lack a default a value

Open points:
- Finish MySqlDefaultChangerTest
- Revise purging logic (#792)
- Allow to have more columns nullable (#814)

src/main/java/fr/xephi/authme/permission/DebugSectionPermissions.java

@@ -29,6 +29,9 @@ public enum DebugSectionPermissions implements PermissionNode {
     /** Permission to view data from the database. */
     PLAYER_AUTH_VIEWER("authme.debug.db"),
 
+    /** Permission to change nullable status of MySQL columns. */
+    MYSQL_DEFAULT_CHANGER("authme.debug.mysqldef"),
+
     /** Permission to view spawn information. */
     SPAWN_LOCATION("authme.debug.spawn"),