Adding a 'dark mode' base64 version of BBC_BLOCKS

[RichardPK]

Part 1/3 to resolve #7185

The first of three PRs to add a dark-mode compatible image placeholder to the psammead-media-player.

1. Add dark mode version of BBC_BLOCKS . (this one)
2. Implement dark mode for psammead-image-placeholder. #3611
3. Implement image placeholder in psammead-media-player. #3600

Overall change:

• Adding a darkMode compatible base64 version of the BBC logo used in psammead-image-placeholder
• Updating story logic to also exclude new BBC_BLOCKS_DARK_MODE base64 js file

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• I have assigned myself to this PR and the corresponding issues
• Automated jest tests added (for new features) or updated (for existing features)
• This PR requires manual testing