[desktop] Add context menu

Summary:
ENG-4407

Adds a right click context menu to the electron app. Uses electron-context-menu for that, because it comes with some nice defaults and window handling.

The github page readme contains a nice explanation of available options, so if anyone thinks we should change the list I'm open to that.

Adding ashoat as there's a new dependency

Test Plan:
Right click on text and images and check if context menu is displayed:

I checked if all visibile options work as expected.

On windows it should work the same way, it will just show an additional "Select All" option.

Reviewers: kamil, inka, ashoat

Reviewed By: inka

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8573