Auto-add prep MR links to issue descriptions

After creating a preparation MR, it will update its associated release issue with the link to itself by replacing either the {{CE_PREPARATION_MR_LINK}} or {{EE_PREPARATION_MR_LINK}} values.

This eliminates one bit of manual toil during the prep MR process.

However, after finally debugging an annoying issue with this, I noticed it may not even be necessary to begin with! Because the prep MRs link to their "parent" task issue, they're automatically added to the shiny new "Related Merge Requests" section of the issue, so we may not need to include the MR link in the issue body at all.

See gitlab-org/release/tasks#679 (closed) for an example of both.