Ask user for previous polling station end time when changing stations

[aniri]

When the user clicks on the change station button:

• a dialog should show up asking them to fill in the leaving time for the polling station they are leaving from
• call PUT /api/v1/polling-station and send leaving time for old station
• go to pick polling station screen
• send new polling station data to backend

Ideally the flow would be this:

• User taps on Change station. We'd show a custom view controller saying "please select the time of departure from the last station" with the time picker that we use in the Station Picker screen. When user taps Done, we upload that information to the server then proceed to the Station Picker screen.

[CristiHabliuc]

@aniri should we also rename the Change button to Leave / Change, so that it's clear that the observers should tap that button when they leave a polling station?