This also adds a new column, message_checksum, to the action_mailbox_inbound_emails table
for storing SHA1 digest of the email source. Additionally, it makes generating the missing
message id deterministic and adds a unique index on message_checksum and message_id to
detect duplicate emails.
Since Action Mailbox will be introduced in Rails 6.0,
it makes more sense to generate migration of that version.
Also I changed its test dummy app to use default 6.0 configs.
