Improve invite logic for existing users

[camdendotlol]

What this PR does

This PR addresses an issue where existing users' logins would break if they were invited via the CSV import tool.

Previously, we checked for the invitation_accepted_at field when deciding whether to re-invite an existing user. But this field is nil for users who created their accounts before the invite system existed. To address this oversight, I've added another check for sign_in_count > 0, so someone who has successfully signed in before won't be re-invited.

How to test

1. Tell me you're about to test this so I can turn on the COVE staging service worker.
2. Go to https://staging.covecollective.org/ and sign in with your main account.
3. Go to one of your anthologies with users in it.
4. Remove a user who has accepted their invitation.
5. Reinvite them with a CSV file.
6. They should a) be re-added to the anthology, b) with a green "Accepted" status, and c) not receive an invitation email.
7. Invite someone entirely new to the anthology with the CSV importer. Make sure to use an email address you can check (e.g. steve+7834879324@performant)
8. They should a) be added to the list, b) with an orange "Pending" status, and c) recieve an invitation email.