Fixed dropdown appearing over sidebar

[asharonbaltazar]

Fixes #594

It adds styling to give the sidebar a higher z-index and a white background-color, thus allowing it to appear over the dropdown.

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[pavish]

Adding a higher z-index to the left pane would fix this issue but would introduce other issues such as the dropdowns in header (schema selection dropdown) would be positioned behind the left pane.

This issue is a side-effect of the dropdown attaching by default to the body. This ensures it always appears over other elements.

There are two ways, this can be resolved:

1. For this particular case, the dropdown can be attached to the table wrapper or the header element. We can specify the parent (header) as reference to the portal action. This has to be passed down to the dropdown and then to the portal action.
2. The simpler approach, which I recommend: Hide the dropdown when column header is not visible. It can be done using css. Refer: https://popper.js.org/docs/v2/modifiers/hide/

Either of these approaches should only be applied on the column dropdowns, and not commonly to the dropdown component.

@asharonbaltazar Please let me know if you'd like to/have time to resolve this. If not, we can close the PR, and come back to it later.