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