Cleanup: drop the `email` column on the `users` table

[carols10cents]

Can only be done after #1888 is fixed. That issue is to remove all uses of email field on User from the Rust code.

What needs to be done:

• Generate a migration using the diesel CLI (search this guide for "diesel migration generate" for more instructions)
• In up.sql, drop the email column from the users table.
• In down.sql, leave a comment that this migration isn't really reversible since we're deleting data like this migration has, but also restore the column with nulls.
• Run the migration and commit the schema changes along with the new migration.