Add migration of D6 database

Problem/Motivation

Users and Content should be available.

Steps to reproduce

Proposed resolution

Migrate from from D6 database into D10 database

Remaining tasks

- [✓] User migration
- [✓] User picture migration
- [] User profile migration

- [] Book migration
- [] Comment migration
- [✓] File migration
- [✓] Vocabulary migration
- [✓] Taxonomy term migration

- [] Node type audio migration (42 nodes until 2013)
- [] Node type bild migration (170 nodes)
- [] Node type drupalchannel seite migration (9 nodes until 2010)
- [] Node type drupalchannel termin migration (54 nodes until 2011)
- [] Node type drupalsite migration (27 nodes until 2010)
- [] Node type dug migration (1 node)
- [] Node type event migration (6 nodes)
- [] Node type faq migration (77 nodes)
- [✓] Node type forum migration (51477 nodes)
- [] Node type handbuch seite migration (650 nodes)
- [] Node type news migration (630 nodes)
- [] Node type page migration (70 nodes) -> news
- [] Node type poll migration (15 nodes)
- [] Node type screencast migration (10 nodes)
- [] Node type showroom migration (1250 nodes)
- [] Node type stickynote migration (7 nodes)
- [] Node type story migration (5 nodes)
- [] Node type team mitglied migration (20 nodes)
- [] Node type uebersetzung migration (1 node)
- [] Node type verlosung migration (10 nodes)
- [] Node type webform migration (3 nodes)

User interface changes

API changes

Data model changes