API /projects/:id/merge_requests is triggering parsing of participants which may not be needed

Per Grafana the API /projects/:id/merge_requests appears to trigger the whole Markdown parsing pipeline to get a list of participants out, yet it doesn't seem to actually expose this data anywhere. Unless this is absolutely required this should be removed.

So instead expose Merge Requests as MergeRequestBasic that does not include properties that require extra load.

Do the same for /projects/:id/issues using IssueBasic.