Use Immutable Releases #2920

There's a checkbox in the settings Enable release immutability: we should check it.

The docs are just vague enough that I'm not 100% sure if our current released workflow works with it:

• Description is "Disallow assets and tags from being modified once a release is published"
• We don't modify assets but do add them after the initial publish: I think that might still be a violation

Maybe best to tweak the release workflow so the initial publish makes the release a draft (and the final publish makes it non-draft): then we should be good to go.