improve background image loading

[RyanCoulsonCA]

Changes

• Background images are converted to a BLOB after first load and is cached. Should hopefully make background image loading faster.
• I've also increased animation times by 0.1 to hopefully look a tad bit smoother.

Testing

Steps:

1. I've temporarily included the TCEI-IECT product in this PR. You can access it by changing the 00000000-0000-0000-0000-000000000000 to TCEI-IECT in the demo URL.
2. Ensure images are transitioning smoothly.

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This change is 

[github-actions]

Your demo site is ready! 🚀 Visit it here: https://ramp4-pcar4.github.io/story-ramp/bg-fix-1/#/en/00000000-0000-0000-0000-000000000000

[spencerwahl]

Reviewed 99 of 99 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @RyanCoulsonCA)

[yileifeng]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @RyanCoulsonCA)

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public/TCEI-IECT/TCEI-IECT_en.json line 1 at r1 (raw file):

{

Guessing this needs to be removed before merge

[szczz]

Reviewed 99 of 99 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @RyanCoulsonCA)

[yileifeng]

Reviewed 1 of 99 files at r1, 98 of 98 files at r2, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @RyanCoulsonCA)

[yileifeng]

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