Enable Statick markdown check in CI.

[tdenewiler]

As part of #508 we have to disable the checks using the Statick markdown plugin. That is because the version of Statick in the #509 PR that deals with #508 uses a new plugin mechanism that Statick markdown will not support yet. We have to update Statick, then Statick markdown in sscpac/statick-md#72. After Statick markdown is updated in sscpac/statick-md#72, we need to come back and re-enable that plugin for the Statick CI workflow.