[web] Turn InvalidUploadModal into a functional component

Summary: Was looking at this as part of the work to create a common Alert/Prompt component and noticed it might as well be a functional component... so made that change.

Test Plan:
Dragged a random non-image file into the chat area and the InvalidUploadModal component was displayed as expected:

Reviewers: palys-swm, def-au1t

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4206