Improving electron security for our service

[phongcao]

The following behaviors have been added to improve electron security:

• Enabling contextIsolation, which allows scripts running in the renderer to make changes to its javascript environment without worrying about conflicting with the scripts in the electron API or the preload script.
• Blocking mouse middle-clicking, which opens a new window and makes our remote input stop working.
• Disabling popup dialog for file downloading (when clicking on a link) so that it doesn't interfere with the streamer window.
• Displaying warning about using insecure http protocol or when the streamer window navigates to a new origin, which is different from the SERVICE_URL.

This will fix #17 after landing.

[phongcao]

@bengreenier Can you do a second round review?