feat: change unique idx on messsaging table

[raquelmsmith]

Problem

In #17365 we want to change the messaging record so that we can send multiple emails with the same campaign key, but only on some sort of frequency. We still need to make sure the campaign send number is is unique though so we don't try to send multiple emails at once.

Problem with that PR is that we are changing the constraint on the table, and changing constraints can have issues. So, my understanding is that we need to:

1. Create a new index with the constraint including the new field (this PR)
2. Drop the old constraint in feat: add field to never drop data for a customer #17365

Changes

Adds a new field campaign_count to the MessagingRecord model.
Creates a new index concurrently with a constraint including that field.

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How did you test this code?

[raquelmsmith]

@frankh I think this is what you had in mind but lmk if not. The existing PR will need to be updated to drop the old index.

One question I have is around Django's makemigrations command. If I change this on the messaging record:

class Meta:
        unique_together = ("email_hash", "campaign_key")  # can only send campaign once to each email

Then makemigrations will notice that it has been changed and will try to create a migration with AlterUniqueTogether. But in creating / dropping the indexes manually that won't be necessary.... so do I remove this line completely? Then we have no record of the uniqueness in the codebase, aside from the migrations.

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