Add undoHistory and redoHistory for existing accounts.

Some older accounts don't have the undoHistory and redoHistory properties, so
they have to be added. This commit checks the loaded data object for those
properties and initializes them with an empty array if neccessary.
Mimic async behaviour in mocked backend service.

The mocked backend service should behave like the real backend service
implementation. This means that the returned promise should be resolve
asynchronously during the next tick, instead of synchronously in the same tick.
Move the marking of entries as volatile to service.

The code which marks entries as volatile when they have been saved to the
service but not yet persisted to the backend is now part of the PasswordService
instead of the PassListController.