[web] Use Alert component in InvalidUploadModal

Summary:
"Consume" the Alert component that was introduced in the previous diff.

Depends on D4207

Test Plan: Dragged a random non-supported file into the chat area and observed that the InvalidUploadModal looked as expected:

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4208