Consistent classNames for outer component containers

[wbrgss]

Because CSS has a top-down, one-way flow for selecting HTML elements, if the outer containers of components do not have a permanent className in addition to one set by the user, it makes it difficult to select the containers themselves or even (conditionally) their nested elements (even if these do have classNames).

For example, this commit https://github.com/plotly/dash-core-components/commit/5aa0ea888c389f5d4d28e06213a730214c65a62e set a className on the dcc.Dropdown outer container, which makes it easier to select with CSS. It would be nice to do this in a consistent way for other components.

This was done well for another Dash library in https://github.com/plotly/dash-daq/pull/80#discussion_r387853372 — seems like a good PR to draw inspiration from.