Fix HTML generation error

[obulat]

These changes have been merged as part of PR #252.

Fixes

Fixes #204 by @panchovm

Description

The chooser currently uses .svg files in the richtext field for license attribution. When you copy the text and paste it, you get 2 outcomes:

• If you paste to a text editor, you get the text of the license icon titles (This work is licensed under CC BY-SA 4.0CC iconby iconsa icon)
• If you paste it to a word processing application (I tried MS Word Online and Google docs), you get empty squares, because pasting SVG format is not supported.
  This PR enables users to copy and paste the richtext license attribution with the icon images.

Technical details

This fix is accomplished by removing the title and alt attributes from the license icon images to prevent pasting plain text and replacing SVG format with PNG format, which word processing apps can paste.
This PR also cleans up the assets folder from unused files.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main or master).
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

Developer Certificate of Origin

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[obulat]

All of the changes introduced in this PR have actually already been merged as part of #252.. Here, I've added the changes to HTML generation to ensure that the generated HTML has the correct text. Previously, after we've changed the generated text for CC0 ('marked' instead of 'licensed'), the HTML generation has not been updated, and it was generating a phrase like 'This work CC BY' instead of 'This work has been licensed under CC BY'

[obulat]

Closing this as the changes have already been merged.