Apply Rails/InverseOf rubocop rule

Not doing so has a few gotchas when working with relations, particularly
with records which are not stored in the database.

I'm excluding the related content file because it's got a very peculiar
relationship with itself: the `has_one :opposite_related_content` has no
inverse; the relation itself is its inverse. It's a false positive since
the inverse condition is true:

```
content.opposite_related_content.opposite_related_content.object_id ==
  content.object_id
```

app/models/poll/question.rb

@@ -11,12 +11,15 @@ class Poll::Question < ApplicationRecord
   belongs_to :poll
   belongs_to :author, -> { with_hidden }, class_name: "User", inverse_of: :poll_questions
 
-  has_many :comments, as: :commentable
+  has_many :comments, as: :commentable, inverse_of: :commentable
   has_many :answers, class_name: "Poll::Answer"
-  has_many :question_answers, -> { order "given_order asc" }, class_name: "Poll::Question::Answer", dependent: :destroy
+  has_many :question_answers, -> { order "given_order asc" },
+    class_name: "Poll::Question::Answer",
+    inverse_of: :question,
+    dependent:  :destroy
   has_many :partial_results
   has_many :pair_answers
-  has_one :votation_type, as: :questionable
+  has_one :votation_type, as: :questionable, inverse_of: :questionable
   belongs_to :proposal
 
   attr_accessor :enum_type, :max_votes, :prioritization_type