Wagtail-modeltranslation

* Adds support for Wagtail 2.0;
* Create new `migrate_translation` management command that runs migrations and `sync_page_translation_fields`;
* Fix slug auto-population for translation with dash;
* Make localized `StreamField` optional;
* Fix `change_lang` template tag to support multiple parameters;

- added RichTextFieldPanel to the default list of patched panels
- added settings to allow the patching of custom panels
- slug auto-population is now made the same way as wagtail (no changes in live pages)
- Fixed: When adding a page link in a translated RichTextField the link was always to the default language version of that page
- Fixed: Copy content of streamfield fails with 414 Request-URI Too Long
- Fixed: Panel patching failed with the error "AttributeError: 'list' object has no attribute 'children'"