Feature/comp 182 improved nren selection

• added the country to the nren table with a datamigration (the datamigration also removes 6 non-member nrens from the database)
• also add this country to the datapoints in the endpoints
• update the endpoint schema's and lowercase all properties in the endpoints where that wasnt the case yet
• use the country in the nren filter popup
• style the nren filter popup and the filter buttons somewhat more according to the design
• fix the bug that multiple selected years were possible with max1year set when navigating from another page
• first code quality pass over the frontend:
  • renamed data-analysis page to budget
  • reduced code duplication for getting data from the api endpoints
  • fixed a warning and code duplication in the policy page
  • removed unused stuff