[fix+mod] in Representative, move local_party to a ManyToMany and remove local_party_role
[fix+mod] in Representative, move local_party to a ManyToMany and remove local_party_role

local_party_role was a duplicated foreignkey instead of a charfield

We choosed to use a ManyToMany because this could help us to track the
changements of party of a representative.

yohanboniface/memopol-core,yohanboniface/memopol-core,yohanboniface/memopol-core