[gui] build-time config

[dskvr]

build time config to set various variables, these values should not remain hardcoded.

• preferred monitors (for onboarding and initial load)
• default nip-66 relays (for onboarding and initial load)
• enable wsproxy (abililty to turn off sertvice-workers)
• enable imgproxy (abililty to turn off service-workers)
• wsproxy url (can be overridden by User Preferences #194)
• imgproxy url (can be overridden by User Preferences #194)
• +more