feat(massEmails): allow one-offs to be re-sent

[rgraber]

🗒️ Checklist

1. run linter locally
2. update all related docs (API, README, inline, etc.), if any
3. draft PR with a title <type>(<scope>)<!>: <title> TASK-1234
4. tag PR: at least frontend or backend unless it's global
5. fill in the template below and delete template comments
6. review thyself: read the diff and repro the preview as written
7. open PR & confirm that CI passes
8. request reviewers, if needed
9. delete this section before merging

📣 Summary

Allow one-off emails to be resent if they have completely finished sending.

💭 Notes

Once a one-off email has records enqueued, set the 'live' flag to read-only so it can not be set to False and then re-added until all enqueued records have been sent. Once all records are sent for a one-off email, set live to False so it is clear that it is no longer sending and allow users to re-add it to the send list another day.

👀 Preview steps

Use mailhog or file-backed email send for easier testing.

1. ℹ️ have at least superuser account and one other account
2. Make sure MASS_EMAILS_CONDENSE_SEND is True
3. Set MAX_MASS_EMAILS_PER_DAY to 1
4. In admin, add two user emails to Constance > Config > MASS_EMAIL_TEST_EMAILS
5. Create a new MassEmailConfigs, with the 'test_users' query and a frequency of -1. Don't check 'live'
6. Add the new config to the daily send using the admin action
7. The new config should now show as live in the list view
8. 🟢 At the next 15-minute boundary (:00, :15, :30, or :45) (give or take a minute), the first user should receive the email
9. Before the next 15 minute boundary, go to the config in Django admin
10. 🟢 The 'live' field should be read-only
11. Try to add the config to the daily send again using the admin action
12. 🟢 You should get an error message "Emails for {config.name} have already been scheduled"
13. 🟢 At the next 15 minute boundary, the second user should receive the email
14. Go back to the config in Django admin. 'live' should now be unchecked
15. Try to add the config to the daily send again
16. 🟢 The config should be successfully added to the next "day"'s send