#792 #814 Implement SQLite migration, allow last IP to be nullable in MySQL

- Old SQLite setups have the last IP column as NOT NULL but without a default value. With the new concept (where we don't set a last IP on player registration) it fails.
  - Create an /authme debug child that allows to migrate SQLite (tricky because SQLite does not support dropping or modifying columns)
  - Allow last IP column to be NOT NULL in MySQL as well (extend MySQL /authme debug child)
- Add TODO comments with follow-up issue to extend our commands with new registration IP field

src/main/java/fr/xephi/authme/datasource/MySQL.java

@@ -192,7 +192,7 @@ public class MySQL implements DataSource {
 
             if (isColumnMissing(md, col.LAST_IP)) {
                 st.executeUpdate("ALTER TABLE " + tableName
-                    + " ADD COLUMN " + col.LAST_IP + " VARCHAR(40) CHARACTER SET ascii COLLATE ascii_bin NOT NULL;");
+                    + " ADD COLUMN " + col.LAST_IP + " VARCHAR(40) CHARACTER SET ascii COLLATE ascii_bin;");
             }
 
             if (isColumnMissing(md, col.LAST_LOGIN)) {