[web] Introduce minimally viable Alert component

Summary:
At this point pretty much copied from InvalidUploadModal with title and children passed in as props. In the next diff will "consume" the Alert component in InvalidUploadModal

Depends on D4206

Test Plan: Will be tested implicitly by the next diff in the stack when we "consume" the newly introduced Alert component from InvalidUploadModal

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4207