Easily replace links in feature flag template

What does this MR do and why?

When creating a new feature flag issue, the template adds several links scattered throughout the description. These links should point to the main issue that the feature flag is aiming to solve. However, the template does not mention it in the comments so it's likely that the user creating the issue leaves those links broken. It's also time consuming and error prone to manually search and replace all those placeholder links.

To solve this I'm proposing to use a markdown link ID that the user sets once, at the beginning of the issue description, and it will automatically replaced everywhere needed.

This change stemmed from a conversation on Slack.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.