Allow moderators to post in a thread even if they already had an anonymous identity

[essential-randomness]

Currently there is no way to post in a thread with a different identity if you already had one assigned. For moderation purposes, it's possible that a moderator will need to make an official post on a thread they already joined. This should be possible without them revealing their previous identity.

On a technical level, this will mean creating a table that assigns a per-post identity to a thread on the backend and returning a different secretIdentity for certain posts when displaying the thread. On the frontend, we shouldn't need to make a lot of changes, except that we might need to show the role selector dropdown even after an identity is already taken. We will likely also need to create a permission for some roles to be used as additional identities in a single thread.

[essential-randomness]

This is the current design document for this feature: https://docs.google.com/document/d/1ChrzMKpOveii8SljZy0aGl9OzfUkGq9IADokrSzspR8/edit#