Add Progressive Web App support

[koensayr]

• Add initial support for Progressive Web Apps
• Primarily Allows iOS Devices to display Poster properly in full screen
• Setups up a very conservative offline cache (not really useful in the Posterrs case anyway)
• Needs deeper testing

[koensayr]

Adopted and implemented based on the guide here

[petersem]

• Add initial support for Progressive Web Apps
• Primarily Allows iOS Devices to display Poster properly in full screen
• Setups up a very conservative offline cache (not really useful in the Posterrs case anyway)
• Needs deeper testing

Doesn't a PWA require either a https or localhost address? So unless you run a reverse proxy and cert, im not sure it world work.