add support for transitional background images

[RyanCoulsonCA]

Related Item(s)

#401

Changes

• Adds support for transitional background images. The configuration flag is backgroundImage and takes a path to an image.
• When a background is active, text components (text slides, captions, titles, etc.) will be placed in a semi-transparent container to keep an accessible contrast.

Testing

Steps:

1. Open the sample page and scroll down.
2. Ensure that contrast is OK for text, captions, titles, etc.
3. Ensure that everything is working well on mobile.

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This change is 

[github-actions]

Your demo site is ready! 🚀 Visit it here: https://ramp4-pcar4.github.io/story-ramp/fix-401/#/en/00000000-0000-0000-0000-000000000000

[yileifeng]

Reviewed 11 of 11 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @RyanCoulsonCA)

[KashishMistry]

Reviewed 11 of 11 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @RyanCoulsonCA)

[szczz]

Looks great, I might have missed it, but is the text colour in the text components configurable? The lack of contrast between black text and darker images likely violates some accessibility rules / UI standards.

Reviewed 11 of 11 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @RyanCoulsonCA)

[RyanCoulsonCA]

I set the opacity of the background such that the contrast should be fine (and the few I tested did pass), but either way I think adding an option to configure the colour would be a good idea. I'll add that in!

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @RyanCoulsonCA)

[RyanCoulsonCA]

I've added a config option to dynamic and text panels to customize the colour of the text. There's a minor issue with bolded text still showing in black (as a result of the prose class), but we can make a new issue for this. I've also updated the schema to reflect the config changes made in this PR.

Reviewable status: 8 of 12 files reviewed, all discussions resolved (waiting on @KashishMistry, @szczz, and @yileifeng)

[szczz]

Reviewed 4 of 4 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @RyanCoulsonCA)

[yileifeng]

Reviewed 4 of 4 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @RyanCoulsonCA)