Django 1.7 support (take 2)

All the bugs blocking the previous attempt to add Django 1.7 support seem to be fixed now.

Wagtail has moved on so much since then though, so this pull request starts adding Django 1.7 support again from scratch.

- [x] Create initial schema migrations
- [x] Create initial data migrations
- [ ] Update ElasticSearchQuerySet to use Django 1.7 where nodes
- [ ] Get all tests working
- [ ] Test