[desktop] Code signing for disk image

Summary:
This diff configures electron-forge to sign and notarize .dmg files.

Additionally I'm using the fileicon utility for changing the .dmg icon. I think having default macOS dmg icon is the standard so there's no explicit option for it in the electron-forge.

Test Plan: Test if running development version works. Check if making the .dmg works (yarn make --platform darwin).

Reviewers: tomek, atul, ashoat

Reviewed By: tomek, atul, ashoat

Differential Revision: https://phab.comm.dev/D5825