Add sidebars

[cpsievert]

See the NEWS.md for a summary of what this PR does.

Closes #480 (as a side-effect)

TODO

• Automated tests
• Add bindCache() to flights demo

TODO (in follow-up PRs)

• Add fillable = fill argument to shiny::uiOutput(). Deprecate "container"/"item" value.
• Add a card_body_fillable() alias for card_body_fill() (and make the latter @Keywords internal?)
• Add a page_fillable() alias for page_fill()?
• Get rid of fill argument in layout_column_wrap() and instead add that behavior as a heights_equal option?
• Consider renaming fillable to fillable_panel once we've renamed nav() to nav_panel() (Add nav_panel() alias for nav() and navset_*() aliases for navs_*() #476)