Store email hashes and signatures in db and run GCD public key finder for missing keys

[foolo]

Created new issue from "Part A" here: #70 (comment)

Most of this is already done, so this issue is mostly for better overview.

Steps needed:

• (Done: 350e709 ) Create a new database table "EmailSignatures", with columns: Domain, Selector, HeaderHash, DkimSignature. (Edit: also need a Timestamp column, as per feedback below)
• Implement/find a tool for canonicalization of email headers during upload. Maybe we have code for this in some of the other zkemail projects. (done, using 3rd party lib for now, but see Migrate from 3rd party canonicalization lib to zk-email-verify helpers #93 )
• (Done: 350e709 ) When a user uploads an email via /upload_gmail: Canonicalize header fields and calculate hash. Put the hash in EmailMessages table.
• (Done: foolo/sigs2rsa@3c1574b ) A small modification of the current GCD solver (https://github.com/foolo/sigs2rsa/blob/main/sigs2rsa.py) to simply use those hash signatures instead of calculating them from plaintext.
• (Done: 350e709 ) Update the privacy policy with new info about data usage. (Do we need a re-verification with Google for that?)
• (Done: 06aba0b ) (If we choose to implement only part A: Create a script that runs the GCD solver for new records in EmailSignatures table, and adds any new keys to the db.)

[foolo]

done, see references in 1st comment