Add support for landing page images in image / featured blocks

[richardTowers]

What

Adds support for "landing page" images, which are a different size to the default images in whitehall.

These can be used in two types of "block" - the image block, and the featured block. The code to present these blocks is updated to transform: [Image: blah.png] to the appropriate asset URLs, as we do for the hero blocks.

Largely for consistency with hero blocks, the config we're expecting users to write is:

sources:
  desktop: "[Image: blah.png]"
  tablet: "[Image: blah.png]"
  mobile: "[Image: blah.png]"

However, as the aspect ratios of each image are the same (just different sizes), rather than adding three image kinds with two variants (1x / 2x) each, I've just added one image kind with six variants. Since we don't need or want to do art direction, I expect that we'll just upload one image and use it for all three sources (desktop / tablet / mobile).

It might be better in future just to do source: instead of sources: for these images, but I think that's a change which will likely confuse us at this point, so I'm sticking with the same structure as heros.

I'll update our blocks documentation once this merges.

Screenshots

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[KludgeKML]

LGTM (probably needs whitehall team approve too though)

[ChrisBAshton]

LGTM, but one thing threw me: