make the 'graphic content blurring' explainer per-organisation

[twrichards]

We need different organisation specific explainers for the 'graphic content' blurring, since it's 'opt-out' (see https://github.com/guardian/grid/pull/4187).

So based on the existing config property branding.staffPhotographerOrganisation, it loads the relevant HTML file from the new public/js/common/blurring directory (e.g. staffPhotographerOrganisation is GNM so loads GNM-explainer.html from that directory. This is rendered on top of the translucent overlay and the GNM implementation references the graphic-image-blur-explainer__tooltip class as before but allows for completely different styling/layout if needs be).

From a GNM perspective this is a no-op (which can be verified in TEST by deleting the SHOULD_BLUR_GRAPHIC_IMAGES cookie and reloading to reveal the explainer).

[github-actions[bot]]

Deploy build 12112 to TEST

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- [Deploy build 12112 to TEST](https://riffraff.gutools.co.uk/deployment/deployAgain?project=media-service%3A%3Agrid%3A%3Aall&build=12112&stage=TEST&updateStrategy=MostlyHarmless&action=deploy) - [Deploy parts of build 12112 to TEST by previewing it first](https://riffraff.gutools.co.uk/preview/yaml?project=media-service%3A%3Agrid%3A%3Aall&build=12112&stage=TEST&updateStrategy=MostlyHarmless)

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From guardian/actions-riff-raff.

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