Number of Claimants - DO NOT MERGE YET

This is a big PR - number of claimants - lots of front end and back end changes.

The main changes are:
 - claimant_module.coffee to handle the expansion/hiding of the claimant block, and validation of num claimants
- renaming claimant_one, claimant_two to claimant_x where x is 1, 2, 3 or 4
- implementation of a ClaimantCollection class which holds the Claimant objects
- changes to all the user_journey specs
- addition of user journey 15 (JS and Non-JS) to test 3 claimants
- addition of continuation sheet on the PDF to hold details of claimants 3 and 4 if present
Is it worth using `MAX_CLAIMANTS` rather than hard coding 3&4 here?