Merge user accounts

[allanlasser]

Now that we can merge organizations, we have a solid understanding of the complexity around merging user accounts. This will require:

1. merging two users in Accounts, with a limitation on accounts that have active subscriptions.
2. sending a webhook notification to connected clients, indicating that a merge occurred.
3. merging the related accounts in each connected client. For us, this means DocumentCloud and MuckRock Requests.

When we worked on merging organizations, it took about two months from start to finish. We can expect this will take a similar amount of time.