#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/process/join/AsynchronousJoin.java

@@ -177,6 +177,8 @@ public class AsynchronousJoin implements AsynchronousProcess {
         });
     }
 
+    // TODO #792: lastlogin date might be null (not updating now because of has_session branch changes)
+
     private boolean canResumeSession(Player player) {
         final String name = player.getName();
         if (database.isLogged(name)) {