Show warning for markdown structure changes

What does this MR do and why?

Issue: #338937 (closed)

• Show a warning in content editor when on editing a document, the markdown style of the original document might not match the resulting document.
• Remove the content_editor_block_tables feature flag.

Screenshots or screen recordings

How to set up and validate locally

1. Create a new wiki page

2. Enter the following content:

   |This is a |table|
   |----------|-----|
   |with <ul><li>hi| there|

3. Edit something in the table.

4. The warning appears.

If you dismiss the warning, it never appears again unless you refresh the page.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.