Prevent clicking Related Post links in the editor.

[JakePT]

Description of the Change

Prevents clicking on links in the Related Posts block in the editor.

Originally I had planned on mimicking the behaviour of the core Latest Posts block, and the code is based on that implementation, which displays a notice when attempting to click a link. However, on investigating more core blocks this behaviour is unique to the Latest Posts block and all other blocks, such as the various query loop blocks like the linked post title or date, simply silently preventDefault on any links. To avoid the overhead of needing to manage a notice I've gone with the more common convention.

This PR also converts the Related Posts block's edit component to a functional component.

Closes #2967

How to test the Change

1. Add a Related Posts block to the editor.
2. Attempt to follow the link for one of the results.
3. The link should not be followed.

Changelog Entry

Fixed - Clicking a Related Posts link while in the editor will no longer follow the link.

Credits

Props @JakePT

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.