Overview: ensured stats data is available for stores too

The data was only being passed in places where a table is displayed, and
since no table is displayed for stores, they were missing data and were
provoking a JS syntax error.
Fixed copyright year
Docs: Switch to https for external links where available
Sprite: restored error icon

Fixes #288.
Print info message after refresh_stats command has been run
Log cache missed as error
Return empty result when we get cached values not `from_update`

We can try to get cached_values when:

- getting stats for overview pages;
- calculating stats for VirtualResources (non-cached TreeItems) when
  display overview pages too;
- updating cached values in a RQ jobs;

Let's split these cases with `from_update` parameter.
Fix #293, #294
Use VirtualTreeItem (non-cached) class as a base class for VirtualResource
Change doc strings