Allow any type of report to store archives of reports and always show the history regardless of type

[brendanheywood]

Split from #93

When I swapped it back to scheduled the history returned (phew!) but it would be better if it never went away in the first place. It would be nicer if any report could optionally have history, like run an an hoc report and then tick another box saying 'Save the results an archive'.

[timhunt]

As comented in #93, I am not convinced.

[brendanheywood]

The reason I split this task is I see it as independent of scheduled reports. I want this feature for on demand ones too, so I can just run it on demand and save it, so I have an opt-in history of snapshots at different points in time. And in many use cases I really don't want it scheduled as it would add clutter to the results.