Warn user before sending transfer with memo prefixed with #

When a user wants to send an encrypted memo with a transfer, he/she prefixes the memo with the '#' character. But if the user has not logged in using his/her memo key, the memo is not encrypted and sent in clear text.

A warning should be issued to inform the user that

• his/her memo will not be encrypted if going further
• he/she should logoff and login again using his/her memo key (or perform any easier action to do so)