Loading devtools should not cause shiny to be loaded

[wch]

Currently, loading devtools causes shiny to be loaded, but it shouldn't. This is because devtools imports miniUI, which imports shiny.

Searching the code for miniUI, I see only this (and the corresponding entry in NAMESPACE):

#' @importFrom miniUI miniPage

Searching for miniPage, I see no other uses of it. So I think miniUI can be removed as a dependency, and I think this will make it so shiny will no longer be needed at all by devtools.

[jennybc]

Closing in favor of #2524