Introduce a normalizer for conditions

[coorasse]

In a situation with a has_many through or has_one through association, cancancan could produce more joins than necessary if the rules were poorly defined.

e.g.

given the class:

class Article
  has_many :mentions
  has_many :mentioned_users, through: :mentions, source: :user
end

and the following rules:

can :read, Article, mentioned_users: { name: 'luke' }
can :read, Article, mentions: { user: { name: 'jabba' } }

this was producing the following query:

SELECT DISTINCT "articles".* 
FROM "articles" 
LEFT OUTER JOIN "mentions" ON "mentions"."article_id" = "articles"."id" 
LEFT OUTER JOIN "users" ON "users"."id" = "mentions"."user_id" 
LEFT OUTER JOIN "mentions" "mentions_articles" ON "mentions_articles"."article_id" = "articles"."id"
LEFT OUTER JOIN "users" "users_mentions" ON "users_mentions"."id" = "mentions_articles"."user_id"
WHERE (("users_mentions"."name" = 'jabba') OR ("users"."name" = 'luke'))

The rules normalizer recognises this situation and normalizes the rules automatically into:

can :read, Article, mentions: { user: { name: 'luke' } }
can :read, Article, mentions: { user: { name: 'jabba' } }

producing the following query instead:

SELECT DISTINCT "articles".* 
FROM "articles" 
LEFT OUTER JOIN "mentions" ON "mentions"."article_id" = "articles"."id" 
LEFT OUTER JOIN "users" ON "users"."id" = "mentions"."user_id" 
WHERE (("users"."name" = 'jabba') OR ("users"."name" = 'luke'))

The RuleNormalizer can be extended to allow also the usage of STI and solve the following issue: #532