Define content security policy

[benfrancis]

Investigate current console warning:

Electron Security Warning (Insecure Content-Security-Policy) This renderer process has either no Content Security
    Policy set or a policy with "unsafe-eval" enabled. This exposes users of
    this app to unnecessary security risks.

For more information and help, consult
https://electronjs.org/docs/tutorial/security.
This warning will not show up
once the app is packaged.