Block proposal submission from users with unconfirmed e-mails

[caiocestari]

Following up a scenario that happened with Sheila Kimura in 2018 (when she couldn't see her Gravatar picture in the system even though she had linked her account), we realised that user with unconfirmed e-mails were still able to submit conference proposals. This can happen with other users in the future due to typos, and (just having a thought about it) can be a breach for attacks.

This request is to block proposal submissions from users with unconfirmed e-mail addresses.