Removed old Tag and Regid models
Status output for importswid command
Removed encoding argument from prettify_xml

Input to the function should always be unicode. The assumed output
encoding is alwalys UTF-8.
Add tag-file relations in chunks

SQLite does not allow more than 999 parameters to be present. When
adding more than 999 files to a tag using `tag.files = [...]`, it will
fail with an OperationalError.

This commit adds chunking to the `process_swid_tag` function. The
TagFile instances are created in bulk using chunks of 950 items.
Created new apps
Separated tpm app
Moved fields to apps.core.fields
Separated filesystem app
Separated devices app
Separated packages app
Separated policies app
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
Added link from 'first reported' date to according session
Check for existing session in swid inventory view

Show an error message when viewing the inventory view for a device that
has no sessions.

Fixes tnc-ba/strongTNC#158.