Improvement: Stories loading

[CharliePops]

Description

Improve story loading the first time that they are open

Relevant Technical Choices

Prefetch all iframes onload

Unfourtunately, there is no way of doing this reusing one iframe, the iframe blocks the second request(preload).

Testing Instructions

1. Pull the branch improvement/prefetch-story-assets
2. Build and run project the project: npm run build:all && npm run dev:ext
3. Using a VPN connect to location far from Europe/Asias
4. Open privacy sandbox tab on DevTools
5. Open the 'Site Boundaries' menu item on the left
6. Open a story from any card by clicking the story icon
7. Observe: the story animates in without any jump or delay (see screencast for example)

Additional Information:

It's probably better to test this in a Incognito window

Screenshot/Screencast

• Before

story-bad.mp4

• After

stories-working.mp4

Ignore black screen in video, the stories images were not working properly when recording

Checklist

• I have thoroughly tested this code to the best of my abilities.
• I have reviewed the code myself before requesting a review.
• This code is covered by unit tests to verify that it works as intended (NA).
• The QA of this PR is done by a member of the QA team (to be checked by QA).