[web] Add "file" input to imageMenuItem in EditUserAvatarMenu

Summary:
We add hidden input to EditUserAvatarMenu, and have browser file selection dialog appear when imageMenuItem is clicked.

For now we just log the selected file.

Initially we'll kick off image upload/avatar update as soon as an image is selected, but it will probably make sense to introduce some sort of ImageSelectionModal once the most basic flow is complete.

Depends on D8313

Test Plan: Logs selected file info as expected:

Reviewers: ashoat, ginsu, rohan

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8315