forms/pilot: Added NotEmpty validator to email_address field

We should not create accounts without email addresses!
views/flights: Fixed exception in /pinned route

File "/opt/skylines/src/skylines/views/flights.py", line 289, in pinned
  ids = [int(id) for id in ids.split('%2C')]

AttributeError: 'NoneType' object has no attribute 'split'