- remove most of the redundant code
- use Dropdown not ButtonDropdown
- fix layout of menu and button in DataFilesSidebar
- clean up our .dropdown-menu styles
- use variables for arrow and border

`….scss`:
- use class not ID
- position via `top` not `margin-top`
- explain menu position's "magic" value
- explain why positioning is necessary[^1]

`….jsx`:
- use class not ID
use Dropdown not ButtonDropdown[^2]

[^1]: Not desired, but necessary if we use our custom <Button> as is.
[^2]: I don't see ButtonDropdown in latest ReactStrap (v9).

This class is assigned to buttons that:
- **either** have Bootstrap btn-primary
    (in which case the `.workbench-content .btn-primary` styles it)
- **or** are our custom `<Button>`
    (in which case the `composes: c-button` styles it)

This reverts commit 1b644ed.