Features/312 user email digest

This adds a new column 'email_digest' to the User field for collecting statistics and a new optional header 'X-Email' for submit to close off #312.

If the user submits a SHA1 hashed email address, we can use that to uniquely identify a User.

We *only* store either the email address or the nickname, preferring the email address over the nickname.

The query side of this feature is covered in #313