Feature request: allow specifying `feature_mjolnir` rooms in the `config.json`

### Your use case

#### What would you like to do?

I would like to specify `feature_mjolnir` banlist rooms in my `config.json`.

#### Why would you like to do it?

To my knowledge the option doesn't sync or doesn't get stored to account data and I would like to share the same ignore lists across multiple accounts that all share the same `config.json` that I keep copying around.

#### How would you like to achieve it?

I would like to have a block in `config.json` for this.

### Have you considered any alternatives?

I could manually add the room subscriptions everywhere, but that is a lot of effort.

### Additional context

The current situation is a lot of work, so I seldom get around to doing it regardless of having `feature_mjolnir` enabled in my `config.json` and loving it.