Separated core and front apps, removed tncapp

This commit fully removes the old `tncapp` app. Instead, everything is
split up into multiple Django apps in the `apps` directory.
Link to regid view from tag view
Added initial DRF configuration
First simple read-only API for Tags and Entities

Refs tnc-ba/strongTNC#171
Added hyperlinks to serializer
Moved swid api endpoints to swid app
Added core api endpoints