Support Gallery Articles

[JamieB-gu]

Add support for galleries to DCAR.

### Tasks
- [x] Should galleries show an age warning?
- [x] Background colour variations in different parts of the page
- [ ] Logos
- [ ] Labs variant
- [ ] https://github.com/guardian/dotcom-rendering/issues/12292
- [ ] Lightbox
- [ ] Share buttons include image id as fragment
- [ ] Dark mode
- [ ] Apps-specific differences
- [ ] Syndication button hover text colour
- [ ] https://github.com/guardian/dotcom-rendering/issues/12293
- [ ] Use thumbnail images for main media when it's missing
- [ ] Add Rich's design improvements
- [ ] Fix image cropping with long captions
- [ ] https://github.com/guardian/dotcom-rendering/issues/12337

[paperboyo]

Don’t forget the coolest trick in the book: [body] image height is restricted by vh :-)