Preserve source URL in Markdown render output

What does this MR do?

When rendering Markdown using the various Markdown endpoints in the application, the URL specified in the Markdown source is transformed by the Markdown rendering pipeline for several purposes: security, resolving a URL for rendering, etc. The original URL, as stored in the Markdown source, is lost after this transformation.

This Merge Request preserves the original URL, as stored in the source, in the data-canonical-src attribute. The purpose is allowing the Content Editor to re-generate the image or link definition in the client when serializing the HTML back to Markdown.

See #328641 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have not included changelog trailers, because there is no user facing change in this MR.
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• This MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)