add header and footer arguments to navbarPage
remove condition attribute from tabPanel

the tab content visibility needs to be the union of data-display-if and whether the tab is active (the currenty implementation makes it always visible when data-display-if evaluates to true)
Merge branch 'feature/navbar-header-footer' into feature/layout-cleanup
Merge branch 'feature/remove-tab-condition' into feature/layout-cleanup
remove panelWithSidebar (this scenario will be handled by more composable top-level pages)
rename tabsPosition to position
update NAMESPACE For removed panelWithSidebar
functions for creating lower-level bootstrap layouts
move page functions to the top of the bootstrap-layout file
refactor functions to use fixed and fluid explicitly and to introduce the notion of "layout" functions
introduce more layout functions
showcase mode stub controlled by showcase=1 parameter
add validation of layout function inputs
show ui.R and server.R files in showcase mode
use built-in parseQueryString
highlight code with highlight.js
more revisions to layout functions -- go back to page level constructs for fixed and fluid
add fluid parameter to navbarPage to specify fluid vs. fixed layout
add hr tag to exported html builder tags
update NEWS