Make sure users only vote once in the same poll

When skipping verification, we cannot apply the validation rule saying
the document number and document type must be unique, because they'll be
`nil` in many cases. So we were skipping the rule, but that makes it
possible for the same user to vote several times (for instance, once in
a booth and once via web).

So we're changing the scope of the uniqueness rule: instead of being
unique per document number, voters are unique per user. The reason we
made them unique per document number was that back in commit 900563e3
(when we added the rule), we hadn't added the relation between users and
poll voters yet.

app/models/poll/voter.rb

@@ -14,7 +14,8 @@ class Poll
     validates :booth_assignment_id, presence: true, if: ->(voter) { voter.origin == "booth" }
     validates :officer_assignment_id, presence: true, if: ->(voter) { voter.origin == "booth" }
 
-    validates :document_number, presence: true, uniqueness: { scope: [:poll_id, :document_type], message: :has_voted }, unless: :skip_user_verification?
+    validates :document_number, presence: true, unless: :skip_user_verification?
+    validates :user_id, uniqueness: { scope: [:poll_id], message: :has_voted }
     validates :origin, inclusion: { in: VALID_ORIGINS }
 
     before_validation :set_demographic_info, :set_document_info, :set_denormalized_booth_assignment_id