Refactor account lua nginx pieces

[kwypchlo]

From SkynetLabs/skynet-webportal#2010

TODO & DISCLAIMER: that logic only works because the skylink is only set in context where skylink is accessed, unfortunately this assumption is be easy to break if someone without knowledge of this changes an endpoint that otherwise doesn't operate in context of skylink (like registry endpoint) and uses skylink variable for something else - this functionality should be rewritten when refactoring nginx account pieces with tests.