[FR] Add a .no-footer class to add on revealjs slide to hide a globally defined footer

[cderv]

This relates to

• [FR] Ability to hide footer on specific slides including title-slide #1445

For example

##  {background-image="images/screenshot-visual-editor-crossrefs.png"}

::: {.footer}
:::

would become

##  {background-image="images/screenshot-visual-editor-crossrefs.png" footer="none"}

or

##  {.no-footer background-image="images/screenshot-visual-editor-crossrefs.png"}

[cderv]

PR is implementing footer='false'