Use GraphQL to retrieve the history notes for a vulnerability

When viewing an individual vulnerability on the security dashboard, at the bottom of a page is a list of history notes that shows changes to the vulnerability state:

Currently, these notes are retrieved through a REST endpoint. We need to use GraphQL instead. To generate more notes, change the status of the vulnerability at the top of the page using the dropdown:

See Design Issue and parent Epic for details.

Implementation Plan

• frontend Change fetchDiscussions() in ee/app/assets/javascripts/vulnerabilities/components/footer.vue to use GraphQL.
• frontend The page continually polls for new notes. Investigate whether it's possible to use GraphQL with polling, and if so, change createNotesPoll() in the same file to use GraphQL.
• backend As per this comment, look into whether this workaround can be removed after switching to GraphQL.