Prior to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29889,
only the project context were set for the Markdown renderer. For a note
on an issuable, the group context was set to `nil` because
`note.noteable.try(:group)` attempted to get the issuable's group, which
doesn't exist.

To make group notifications work, now both the project and group context
are set. The context gets passed to `RelativeLinkFilter`, which
previously assumed that it wasn't possible to have both a group and a
project in the Markdown context. However, if a group were defined, it
would take precedence, and the URL rendered for uploads would be
`/group/-/uploads` instead of `/group/project/uploads/`. This led to
404s in e-mails.

However, now that we have both project and group in the context, we
render the Markdown giving priority to the project context if is set.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63910