AdditionalClaimantsForm (wip)

A Form object which allows multiple addtional claimants to be created/updated
in one step

Other changes in this commit:

 - Added primary_claimant and primary_respondent to claimants and respondents
   tables respectively, with a default value of false

 - primary_claimant, primary_respondent, secondary_claimants, secondary_respondents
   association on Claim using the aforementioned added columns to delineate

 - Update ClaimantForm and RespondentForm to use these new associations

 - Section urls can now be hypenated, e.g. /apply/additonal-claimants