Create Vulnerabilities::MergeRequestLink to allow linking auto-remediations to Vulnerabilities

Why are we doing this work?

In order to deprecate and stop using Vulnerabilities::Feedback we need to be able to replicate it's functionality of linking Merge Requests with Vulnerabilities

Implementation Plan

• database Create a vulnerability_merge_request_links with a foreign key to vulnerability_id and merge_request_id
• backend Create a model Vulnerabilities::MergeRequestLink with belongs_to :vulnerability and belongs_to :merge_request