Handle singletons and head tags in React context

[kamilzyla]

In Shiny you can use e.g. singleton(tags$head(tags$script(...))) to add some code to document head. This is sometimes used by libraries (e.g. shinyAce, shiny.router) to add dependencies instead of htmlDependency(). However singletons and head tags currently don't work properly in React context.

Note: It is often possible to work around this issue by including the dependencies / mock component manually outside of React context. For example, you can put div(style = "display: none", shinyAce::aceEditor(NULL)) anywhere outside of React to load the dependencies and make aceEditor() work also within React.

[jakubsob]

tags$head and tags wrapped in singleton should be handled in:

• renderReact
• reactContext