Backport of "Track export issues CSV usage on GitLab.com with Snowplow"

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9067

app/views/projects/issues/import_csv/_modal.html.haml

@@ -20,5 +20,5 @@
               = _('It must have a header row and at least two columns: the first column is the issue title and the second column is the issue description. The separator is automatically detected.')
               = _('The maximum file size allowed is %{size}.') % { size: number_to_human_size(Gitlab::CurrentSettings.max_attachment_size.megabytes) }
         .modal-footer
-          %button{ type: 'submit', class: 'btn btn-success', title: _('Import issues') }
+          %button{ type: 'submit', class: 'btn btn-success', title: _('Import issues'), data: { track_label: "export_issues_csv", track_event: "click_button"} }
             = _('Import issues')