PoC: Vertical split for users.incoming_email_token - step 1

DO NOT MERGE

What does this MR do and why?

This is PoC example MR on how to do a vertical split for a database table - extract column(s) and index(es) to a new table.

This is step 1 from the process, that does the following:

• Creates new table user_incoming_email_settings
• Updates the app to:
  • read from the new table
    • with fallback to the original column users.incoming_email_token
  • write to the new table
• Starts back-filling the new table

See #430906 (closed) for more details.