Sessionized email verification

[tomlinton]

Checklist:

• [x] Code contains relevant tests for the problem you are solving
• [x] Code has been formatted with autopep8 --in-place --recursive --a --a .
• [x] Ensure all new and existing tests pass
• [x] Update any relevant READMEs and docs
• [x] Submit to the develop branch instead of master

Description:

• Replace database storage of email and verification code with server side sessions. Email is hashed with a salt in the session
• Cleaned up VerificationCode model
• Added migration to remove unused verification_code table from the database, no more PII being stored!

Resolves issue #46 (for origin-bridge at least, still need to address merkle proof of verification but that is probably more of an issue for origin-js and origin-dapp).