 fix migration for mysql (varchar length is required)
 fix migration for mysql (varchar length is required)

zky001/flaskbb,emile2016/flaskbb,zky001/flaskbb,SeanChen0617/flaskbb-1,qitianchan/flaskbb,lucius-feng/flaskbb,dromanow/flaskbb,China-jp/flaskbb,qitianchan/flaskbb,realityone/flaskbb,dromanow/flaskbb,SeanChen0617/flaskbb-1,zky001/flaskbb,emile2016/flaskbb,emile2016/flaskbb,SeanChen0617/flaskbb,qitianchan/flaskbb,China-jp/flaskbb,SeanChen0617/flaskbb,SeanChen0617/flaskbb-1,lucius-feng/flaskbb,China-jp/flaskbb,lucius-feng/flaskbb,realityone/flaskbb,realityone/flaskbb,SeanChen0617/flaskbb,dromanow/flaskbb