Add LICENSE file

Fixes #333
Add test for country value

The country code value should be conformant to ISO 3166-1 alpha-2. If
it's not, we should get an exception!

Part of #325
Fix db constraints for country value

This adds constraints for making sure the value in the country column is
a two letter alpha character. It is some form of semi-check to verify we
have a ISO 3166-1 compliant value. Doing a proper check towards 3166-1
means we would have to include a list of country codes. We don't want
that as it would likely just become outdated, thus this just performs
the most basic of compliant checks.

Fixes #325.
Fix offset calculation for floating prefix header

The floating prefix header is usually hidden and becomes visible when
the user has scrolled down far enough that the fixed prefix header
is outside of the screen. To achieve this, we get the position of the
fixed header relative to the top and then attach an event handler to
calculate current scroll offset from top.

Problem was that we only retrieved the fixed headers offset from the
screen at page load, and as result is rendered, this could change. Most
notably, the search interpretation will make the search "header" grow a
bit which will change the offset.

This is now fixed by retrieving the position of the fixed header every
time the event handler is called.

Fixes #335.
Merge branch 'done_325-country-field' into develop
Merge branch 'done_333-add-license-file' into develop
Merge branch 'done_335-floating-prefix-header-offset' into develop