Add migration for unique indexes on favourite

[BentiGorlich]

Delete duplicate favorites and introduce 4 new indices.

Fixes #578

[nobodyatroot]

Migration went smoothly, no issues encountered since I stopped the messengers first.

[melroy89]

Did you validate a db migration diff check? So there is no diff created between doctrine and the db? I ask this since you first created the migration and then the doctrine. Instead of the other way around.

[BentiGorlich]

I just derped on pushing the changes. I did generate it through doctrine 😅

[melroy89]

Migration is executed successfully here as well. I think you can merge it @BentiGorlich.

[e-five256]

I wanted to double check the instructions for this migration, as I was unaware of the need to stop messengers during certain migrations. Is that standard for a specific change? Are there instructions on how to do so? Obviously as you've already stated we'll want to make this known in release notes, and just letting peeps know in the matrix general is probably a good idea

[BentiGorlich]

The reason for that is that you might wind up in a deadlock in postgres if you don't stop the messengers (happened to me once with a longer running migration). That is only for long running migrstions though. With this one specifically the migration might fail due to unique violations. Because it is so long running you might get newly created duplicates which then block the creation of the indeces