Shared feed import validation message: Format URLs using Markdown

[caiosba]

Description

When trying to import media from a shared feed that already exist in the target workspace, return the existing item URLs in the error message as Markdown, so they can be better displayed on the frontend.

Reference: CV2-4631.

How has this been tested?

On the frontend side (Check Web branch with same name as this):

Checklist

• I have performed a self-review of my own code
• I have added unit and feature tests, if the PR implements a new feature or otherwise would benefit from additional testing
• I have added regression tests, if the PR fixes a bug
• I have added logging, exception reporting, and custom tracing with any additional information required for debugging
• I considered secure coding practices when writing this code. Any security concerns are noted above.
• I have commented my code in hard-to-understand areas, if any
• I have made needed changes to the README
• My changes generate no new warnings
• If I added a third party module, I included a rationale for doing so and followed our current guidelines

[caiosba]

Requesting review again because I made some changes to format as per conversation with @brianfleming in the ticket.

[DGaffney]

Just make sure to address test failure before merge!