Rework polling stations selection screen

[idormenco]

Polling stations will be identified by CountyCode and CommunityCode.
Add a new dropdown for selecting community in fragment_polling_station_selection.xml
A county contains one or more communities.
To get communities query /api/v1/county/{countyCode}/communities
Polling stations are identified by countyCode and communityCode.
Send communityCode to all requests that contain countyCode.

See rough implementation in #263