Consider using the default Subscriptions footer defined in Tableau footer for VizAlerts

[mcoles]

A user was confused that when they updated the default footer in the Tableau Server site configuration page, VizAlerts did not immediately adopt them. This is because VizAlerts does not make any use of the default footer. But perhaps it should. The work would be pretty easy, as the text is just sitting in the Sites table in the PostgreSQL repository.