Filter headings last on travel advice index

In the travel advice index script (which filters countries) was filtering
the alphabet headings before filtering the actual countries when no filter
query is given. Thus, if the user entered a search term and then deleted it,
the filter did not correctly re-show all the headings.
Configure asset host in development mode.

When run under `govuk_setenv`, this configures the asset host to point
at the assets-origin on the dev VM.  This will give us better dev-prod
parity, and enable running this behind the router in dev.
Merge pull request #630 from alphagov/asset_host_in_dev

Configure asset host in development mode.
Merge pull request #628 from alphagov/fix-travel-advice-filter

Filter headings last on travel advice index