Revert "Revert "Merge pull request #51 from alphagov/show-country-filters""

This reverts commit 4ed1e3bacac4bf668a7c1a11650947101062daf5.
Wrap measures into collection object.

Brings more clarity as measure tables are getting more complicated.
Country measure table does not include third country measures.

As they are included in the table above.
Fix measure sorting.
Exclude measure if filtered country is in its excluded list.

Also do not include measures relevant to filtered country in the table
for specific countries (as they have a separate table now).
Fix specs according to new requirements.
MeasureCollection#present? spec.
Change label of the country filter field.
Remove country-filter notice from overview tab.
Additional formatting for filter info box.