Regarding #38 #39:

- Deleted ballotLocation field in favour of passing ballot filepaths as a parameter to authorize() and provisionalAuthorize() in Model.java
- Renamed 'spoil' back to 'challenge' in most cases regarding Precinct in Model.java and Precinct.java
- Added a pollsClosed() method to Precinct.java to deal with challenging/spoiling committed ballots when polls close
- Cleaned up some code in ActiveUI.java