Prepare for presenting measures for single country.

Extract/move to method import & export measures on DeclarablePresenter
so filtering by country could be done in these two methods.
Add filtering by country to DeclarablePresenter.
Remove stale specs.
Extract search classes for clarity.
Add country select to search form.
Merge search and Tariff Date changer.
Handle searching and date/country changing with one form.
Extract common functionality to ApiEntity.
This model is no longer used.
Move Declarable third country related methods to presenter.

They are used for presentation. Also, they will utilize
DeclarablePresenter#import/export_measures with country filter.

Also adds integration specs for country filter.
Pass down :db formatted date for API search query.
Redudant HTML part.
Add toggledControls to tariff search form.
Use Search#country_name for country name display.
Allow browse date to be specified with as_of param (for compatibility).
Form search styling.
Search#query_attributes should return stringified hash.
Remove TODO, code is not being used.
Merge pull request #45 from alphagov/country_filter

Search form merge and country filter