Show last-modified of tiled layers and DBK features.json

[matthijsln]

Request /0/0/0.png/jpeg and show the Last-Modified header in settings screen.

In Ajax done() function for features.json request, read Last-Modified header and display DBK last updated date in settings screen.

In layout.js, provide function dbkjs.layout.displayDataInfo() to call for other modules/code and move the /bag/info and /infra/info Ajax calls to the bag and infrastructure modules.

[matthijsln]

Last-Modified header for features.json only available for static versions.