allow editors to pick a particular party description for a candidate

[mhl]

We'd like to get the "description" that goes on the ballot paper correct, now that we know what these are going to be from the Statements of Persons Nominated.

When set, this should add a description field to the standing_in entry for the 2015 election; create_candidate_list_memberships should then make sure that's set as a description field on the membership of the post with role "Candidate".

Also:

• The person edit page needs to be updated to allow a description to be picked for the candidate.
• The constituency candidate list needs to display the description if present. (There's probably a discussion to be had about whether we should also display the party name on that list or only display the description.)

Depends on: #314

[mhl]

Note that the descriptions are available via other_names on Organization:

In [3]: o = Organization.objects.get(name='Labour Party')

In [4]: for on in o.other_names.all():
   ...:     print on
   ...:     
The Labour Party Candidate
Labour Party Candidate | Ymgeisydd Plaid Lafur
Scottish Labour Party Candidate
Labour Party Candidate
Scottish Labour Party
Labour and Co-operative Party (Joint Description with Co-operative Party) | Llafur a’r Blaid Gydweithredol
Welsh Labour | Llafur Cymru
Welsh Labour Candidate | Ymgeisydd Llafur Cymru
Welsh Labour Party Candidate | Ymgeisydd Plaid Llafur Cymru
Glasgow Labour
Choose kids, not cuts