Add missing has_many relations for users

Usually when we specify a `belongs_to` relations, we also specify its
equivalent `has_many`. That allows us to write, for example:
`topic.user.topics`.

app/models/legislation/proposal.rb

@@ -20,7 +20,7 @@ class Legislation::Proposal < ApplicationRecord
   acts_as_paranoid column: :hidden_at
 
   belongs_to :process, foreign_key: "legislation_process_id"
-  belongs_to :author, -> { with_hidden }, class_name: "User"
+  belongs_to :author, -> { with_hidden }, class_name: "User", inverse_of: :legislation_proposals
   belongs_to :geozone
   has_many :comments, as: :commentable