[Counties] Extra export options

[aniri]

Depends on #204

The export counties button should be turned into a dropdown picker with the following options:

• export counties only
• export counties and polling stations

Option only allowed for the master admin account (an ngo admin belonging to an ngo that has the organizer flag set to true)

Backend part was implemented in code4romania/monitorizare-vot#253.

UI available here: https://www.figma.com/file/61NHR7fwmV1Dsv4Pb6F8T7/code4ro_mv_user-management_2018.04.21-(Copy)?node-id=754%3A0

Please use bootstrap classes as much as possible and avoid adding custom css styles to keep the codebase clean & consistent. The new UI should be as similar as possible to the one in figma, not pixel perfect.

Also, make sure all texts are extracted in translation files.