Fix sidebar complete showing/hiding on mobile

[bborgesr]

Fixes #127 and fixes #177: previously, if dashboardSidebar() was called with an explicit width parameter, mobile rendering would look weird (the sidebar wouldn't completely disappear when it was collapsed, and content in the dashboard body would be hidden under the still-visible sidebar).

Small example:

library(shiny)
library(shinydashboard)
shinyApp(
    ui = dashboardPage(
        dashboardHeader(),
        dashboardSidebar(width = "150px"),
        dashboardBody("Hello world"),
        title = "Dashboard example"
    ),
    server = function(input, output) { }
)