Kill date pickers

[tcbegley]

This PR removes DatePickerSingle and DatePickerRange as suggested in #148. It appears to have reduced the bundle size by about two thirds (!) from ~750KB to ~250KB.

[tcbegley]

While looking into ramda imports as suggested here I noticed that these two components were also the only places we were using ramda without targeted imports, so possibly a large chunk of the reduction we're getting is due to the blanket imports getting removed.

[pbugnion]

To help people use DatePicker{Single,Range} from dash-core-components, should we write an example in the documentation somewhere? The deprecation exception could then point to that?

[pbugnion]

Apart from that, this looks good.

[tcbegley]

To help people use DatePicker{Single,Range} from dash-core-components, should we write an example in the documentation somewhere? The deprecation exception could then point to that?

We could just link to the dash-core-components docs? I'm not sure that our own documentation would add much?

[ZeroCool2u]

@tcbegley @pbugnion Is there any documentation on how to apply the theme to the DateRangePicker components? I haven't had any luck so far.

[tcbegley]

hey @ZeroCool2u, you can try this https://github.com/tcbegley/dash-bootstrap-css

[ZeroCool2u]

@tcbegley Great, thank you!