Add QuestionVote model

Add QuestionVote model. Include is migration, tests and factory.
Add AnswerVote model

Add AnswerVote model. Includes migration, factory and tests.
Add rank_score

Add rank_score field to Question Model. Include migration.
Add QeustionVoteCountManager

Add QuestionVoteCountManager to create query_set to order rank_score.
Exclude rank_score

Add rank_score to exclude in QuestionForm.
Add rank_score field

Add rank_score field to Answer model. Includes migration.
Add AnswerVoteCountManager

Add AnswerVoteCountManager to Answer model.
Exclude rank_score

Exclude rank_score for AnswerForm.
Merge branch 'vote_model' into develop

Add voting in models.