Add image resizing functionality for perseus file exports

[KshitijThareja]

Summary

Description of the change(s) you made

This PR introduces image resizing functionality to resize assessment images containing resizing data and making sure that the final perseus zip contains resized images.

Manual verification steps performed

1. Creating assessments containing varied image data, like original, resized and duplicate images.
2. Publishing the channel and testing the changes on local Kolibri instance by importing it from the locally running studio server.

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Reviewer guidance

How can a reviewer test these changes?

1. Follow the verification steps listed under the previous heading.
2. Create a dummy exercise and try adding images and resizing them in the exercise questions.
3. Publish the channel and import it in Kolibri(Use local studio for imports by changing the environment variable CENTRAL_CONTENT_BASE_URL to point to the URL of your local studio instance).
4. Checkout the imported assessment to see if images have been resized accordingly or not.

References

Closes #4416

Comments

The approach used here slightly deviates from what was told in the issue description. While working on this issue, I found out that the control flow of the channel publish method never entered the image loop in create_perseus_zip function. So, the image resizing function couldn't be applied there. Instead, the image resizing now happens in the process_image_strings function as this is where the image data is modified originally before being written to the assessment file and it contains the image resizing data too.

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Contributor's Checklist

PR process:

• [ ] If this is an important user-facing change, PR or related issue the CHANGELOG label been added to this PR. Note: items with this label will be added to the CHANGELOG at a later time
• [ ] If this includes an internal dependency change, a link to the diff is provided
• [ ] The docs label has been added if this introduces a change that needs to be updated in the user docs?
• [ ] If any Python requirements have changed, the updated requirements.txt files also included in this PR
• [ ] Opportunities for using Google Analytics here are noted
• [ ] Migrations are safe for a large db

Studio-specifc:

• [ ] All user-facing strings are translated properly
• [ ] The notranslate class been added to elements that shouldn't be translated by Google Chrome's automatic translation feature (e.g. icons, user-generated text)
• [ ] All UI components are LTR and RTL compliant
• [ ] Views are organized into pages, components, and layouts directories as described in the docs
• [ ] Users' storage used is recalculated properly on any changes to main tree files
• [ ] If there new ways this uses user data that needs to be factored into our Privacy Policy, it has been noted.

Testing:

• [ ] Code is clean and well-commented
• [ ] Contributor has fully tested the PR manually
• [ ] If there are any front-end changes, before/after screenshots are included
• [ ] Critical user journeys are covered by Gherkin stories
• [ ] Any new interactions have been added to the QA Sheet
• [ ] Critical and brittle code paths are covered by unit tests

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Reviewer's Checklist

This section is for reviewers to fill out.

• [ ] Automated test coverage is satisfactory
• [ ] PR is fully functional
• [ ] PR has been tested for accessibility regressions
• [ ] External dependency files were updated if necessary (yarn and pip)
• [ ] Documentation is updated
• [ ] Contributor is in AUTHORS.md