Rationalise loan and reservation permissions

- ensure loan_any grants same loan and manual reservation management
  permissions as admin groups in Beaker 0.14
- update docs appropriately
- more systematic testing of expected loan and reservation access
- explicit tests for handling of permission and user validity
  errors in the system loan management web UI

Bug: 1011858
Change-Id: I94bc1939111d9813795aba54e21a4d10c0b2b456