Issues with phantom opinions

Proposed fix to https://www.pivotaltracker.com/story/show/82835486
Involves 2 changes:
  - allowing non-current geo entities in EU Decisions dropdowns, so that if a decision relates to a non-current geo entity, the dropdown is populated correctly
  - setting not null / presence validation consraints on the geo entity field, because it seems it should be in place and it would prevent from creating 'phantom opinions'

Requires:
`rake db:migrate`

Note: the production db currently does not have any EU Decisions without geo entity set, so if the development / staging db has such entries - assume they're incorrect and delete (migration will abort if there are)