Updates for React 16

[tcbegley]

With the release of dash 0.41.0 and dash-renderer 0.22.0, dash-renderer is now using React 16, meaning the work that was happening on the dash-v1 branch can be merged.

Changes include:

• New Modal components + documentation (Add modals #146)
• New Spinner component + documentation (Add spinners #161)
• Allow right-alignment of DropdownMenu (Right-aligned menu #139)
• Update to reactstrap 8

I made a prerelease for testing:

pip install dash-bootstrap-components==0.5.0rc1

[pbugnion]

On modals, I expect people will spend a ton of time writing annoying little callbacks like:

@app.callback(
    Output("modal", "is_open"),
    [Input("open", "n_clicks"), Input("close", "n_clicks")],
    [State("modal", "is_open")],
)
def toggle_modal(n1, n2, is_open):
    if n1 or n2:
        return not is_open
    return is_open

Could we potentially provide a function like:

dbc.interaction.bind_modal_callbacks(app, 'modal')

?

(we could imagine similar callbacks in the dbc.interaction module for other cases where the interaction is obvious, like popovers, fade and collapse).

I don't think that's in scope for this PR, but it might be worth considering.

[pbugnion]

This looks great! Modulo a couple of comments inline, merge whenever!

I think modals and spinners will be a great addition.

[tcbegley]

Thanks for reviewing!

I think your dbc.interaction module idea is nice, definitely worth experimenting with that.