make transparency of crop gutters 50% to make them less 'opaque'

[twrichards]

#4331 added crop gutters under certain scenarios, and we've since been testing with a subset of users (see #4336) and have received the following feedback...

I find the diagonal stripes obscure the sides of the image too much - I can't see what's underneath them to decide if I want to reduce my 5:3 crop

The stripes are white, but with mix-blend-mode: difference which attempts to invert the colour below, however depending on the colour/darkness below this can appear too 'opaque'...

...so this PR is a quick fix to reduce the transparency to 50% (opacity: 0.5) - with hopefully a PR for configurable per-image transparency slider in #4341 - proving a bit fiddly so want to get this one-liner improvement out first, it might even suffice (hehe @paperboyo)

[github-actions]

Deploy build 12819 to TEST

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• Deploy build 12819 to TEST
• Deploy parts of build 12819 to TEST by previewing it first

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From guardian/actions-riff-raff.

[prout-bot]

Seen on auth, usage, image-loader, metadata-editor, thrall, leases, cropper, collections, media-api, kahuna (merged by @twrichards 11 minutes and 34 seconds ago) Please check your changes!