Add assets:regenerate_variants rake task

Regenerates ImageMagick variants from each asset's stored original --
needed because assets uploaded before :headline existed in
FileAttachment::STYLES never had that file generated, producing a 404
rather than a distorted image. Supports ASSET_ID=N scoping so a single
asset can be tested before committing to a full, expensive sweep
across everything.

Doesn't touch the separate, still-open problem of :headline's forced
non-uniform stretch (460x250!) -- this task faithfully regenerates
whatever geometry the style defines, correctly. Fixing the distortion
itself is its own, later piece.

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