refactor(modules): create display, qr, directory modules

[LoneRifle]

Problem

The infrastructure to generate QR codes is poorly organised and too dependent on
calling inversify to resolve its dependencies. This makes it inconsistent in structure
relative to the other controllers and modules in Go.

Whilst we are here, we should move RotatingLinksController into its own module

Part of ongoing work for #869

Solution

• Create display module, move RotatingLinksController into it
• Create qr module, move QrCodeService and assets into it
• Extract QrCodeController from api/qr into modules/qr
• move GoDirectory to module
• Inject config, existing inversify dependencies into classes
  wherever possible

[yong-jie]

Agreed that the qrcode route was exposing too much logic and should be refactored into its own controller. Most of the code lgtm; i'm raising one discussion point to ponder about before changes go in.