Add option for skins to add and overwrite images

[saebekassebil]

This enables MTM (and other skins) to add their own images, and overwrite existing images with new ones.

There's a whitelist integrated which only allows files:

• apple-touch-icon* - (to overwrite the iOS logo)
• /images/* - All image files

This means no scripting and other assets (for now).

This will resolve NOTA-2275 and is a somewhat important issue to get fixed ASAP for MTM.

@m-abs or @dfg-nota How bad does it look? ;)

[m-abs]

Jeg har svært ved, at overskue konsekvenserne af denne pull request.