Add image_url validation in release post item linter

Why is this change being made?

Some images might be committed in the wrong place, and sometimes the image_url is missing the leading slash (/), which results in the final assembly the image being broken.

Since the assembly script seems to be following a strict rule, we can solve this by checking that the image_url always starts with /images/unreleased/.

Tested locally with a wrong image_url:

./bin/validate-release-post-item data/release_posts/13_6/brakeman-severity.yml
error data/release_posts/13_6/brakeman-severity.yml: image_url should start with /images/unreleased/

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