feat: preserve image alt text while converting html to text

[navinkarkera]

Description

Replace image with its alt text while converting html to plain text for indexing.

Useful information to include:

• Which edX user roles will this change impact? Common user roles are "Learner", "Course Author", and "Developer".

Supporting information

• Part of: Text components that contain images with alt text should show the alt text in the description frontend-app-authoring#1670
• Private-ref: FAL-4092

Testing instructions

• Create or edit any html component with image and alt text in library and save it (to reindex it)
• Verify that the card preview shows the alt text in place of the image.

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[rpenido]

LGTM 👍
Thank you for your work, @navinkarkera!

• I tested this using the instructions from the PR
• I read through the code
• I checked for accessibility issues
• Includes documentation

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.