Fix the ban! and unban! methods to:
* Lock the artist while it is being banned or unbanned.
* Perform the edits as a mass update, so that the posts are updated in parallel.
* Edit the artist as the banner rather than as the current user.
* Soft delete the banned_artist implication when an artist is unbanned instead of hard deleting it.
* Ignore the banned_artist implication if it's deleted.
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