Add HEAD placeholder in html template for shiny_prerendered

[cderv]

It should fix #343

This follows changes in

• fix invalid HTML code in shiny_prerendered documents rmarkdown#1942 where we fix a long standing issue with how HTML content was added in shinyapps UI with shiny_prerendered
• then Allow to add specific shiny-prerendered parts in HTML template rmarkdown#2064 which complement the previous one by offering a way to get the html dependencies in the correct order in the HTML template.

I should have thought to update the template in flexdashboard following this change.

It could be a good idea to update the rmarkdown requirement to >= 2.8 but it may not be necessary for this to work. It is the other way around: if you have rmarkdown 2.8 or above you need this version of flexdashboard. Not sure we can do it in this way. If you don't see an issue, I would bump the requirement. (which you may need to do already for bslib support)

[cderv]

About CI, seems there is an issue with htmltools installation. Which also happens in your main CI.
You may already know that.

[cpsievert]

Thanks for the fix and the context! I like the idea of requiring >= 2.8.

[cderv]

Great !

On second thought, you could prefer last rmarkdown 2.9 as minimal version because we fixed a regression in 2.8 and there are some improvement with rmarkdown::run for shiny runtime.