Add filtering by location to statistics

[lvps]

All statistics except "Stats" and "Fix these" need to be filtered by location, i.e. select only items in the Polito subtree.

Users should be able to choose the root with a form in each page, so we can e.g. see stats only for available RAMs in a box instead of every RAM in a pc or in a cabinet or on the floor.

[lvps]

This is partially/mostly done in methods, but still needs a way to select it (an <input>).