FR: UI pageSpinner

It would be great to have a pageSpinner function which can be used to show a pageSpinner on session start - similar to shinyjs::hidden() and shinyjs::hide().

With bigger server functions the showPageSpinner call is delayed, which could be avoided with a client side solution:

```
library(shiny)
library(shinycssloaders)

ui <- fluidPage()

server <- function(input, output, session) {
Sys.sleep(3L)
shinycssloaders::showPageSpinner()
}

shinyApp(ui, server)
```