[web] Add placeholder edit icon to EditUserAvatar

Summary:
This diff adds a placeholder edit icon button icon to EditUserAvatar.

Just handling layout/styling here, will handle functionality separately.

Test Plan:
Looks as expected:

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek, ted

Differential Revision: https://phab.comm.dev/D8242