Use `DOMPurify` to sanitize and mitigate XSS attacks

[TheMythologist]

Description

An XSS vulnerability is present in the Markdown component, where unsanitized user input is directly passed into dangerouslySetInnerHTML. For more info, refer to here.

Note: The changes in UI test snapshots are merely the order of the a tag attributes - they are essentially the same.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update
• [ ] Code quality improvements

How to test

An example would be entering the text <img src="" onerror={alert('xss')} /> in the "Module Help Text" of the Admin Panel page when creating a new course. After sanitization, the alert('xss') command should not be run.

Checklist

• [x] I have tested this code

[coveralls]

Pull Request Test Coverage Report for Build 11007895846

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 31.449%

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Totals
Change from base Build 10982558647:	0.0%
Covered Lines:	4852
Relevant Lines:	14547

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💛 - Coveralls