Added counterparty detail view; merged some transaction classes; update admin for first-party apps

counterparty/:
- Added admin for Alias and Pattern models
- Updated constraints on Pattern model
- Renamed CounterParty.auto_classify to auto_categorise
- Added counterparty detail view (WIP)

finance/:
- Added counterparty/ URL namespace

statementimport/:
- Made processor clean_fields consistent

templates/:
- Added sidebar_page.html base template

transactions/:
- Merged BillPayment, StandingOrder and DirectDebit into single PaymentTransaction
- Updated transaction table templates to show category and counterparty alias