Fix display of activation dropdown confetti

Created by: lguychard

Fixes #8527 (closed)

Two issues here:

• Confetti actually didn't display at all following this change: https://github.com/sourcegraph/sourcegraph/pull/6953/files. Typings in version 0.1.3 of react-dom-confetti are wrong, and passing in numbers for with and height breaks the display of confetti altogether, as the individual divs generated for each confetti have 0px*0px dimensions (https://github.com/daniel-lundin/react-dom-confetti/pull/26/files). The default width/height of confetti looks very similar to '10px' width/height, so I suggest we just rely on the defaults here.

• Z-index of confetti conflicted with other UI elements.