EL-207 Admin interface to create/edit elections

WIP, see also [issue tracker ticket](https://tickets.openaustraliafoundation.org.au/browse/EL-207). TODO:

- [x] Create new party/category creation page
- [x] Create bulk replacement page for australian_postcode & constituency (In a transaction)
- [x] Authentication
- [x] Disable election til it's done
- [x] Split new category/party buttons/page
- [x] Doco for creating new elections