Expire cache when adding documents and images

Proposals and budget investments were not correctly updated when adding,
removing or modifying their documents and images.

app/models/document.rb

@@ -7,7 +7,7 @@ class Document < ApplicationRecord
                                  hash_secret: Rails.application.secrets.secret_key_base
 
   belongs_to :user
-  belongs_to :documentable, polymorphic: true
+  belongs_to :documentable, polymorphic: true, touch: true
 
   validates :title, presence: true
   validates :user_id, presence: true

app/models/image.rb

@@ -12,7 +12,7 @@ class Image < ApplicationRecord
                                  hash_secret: Rails.application.secrets.secret_key_base
 
   belongs_to :user
-  belongs_to :imageable, polymorphic: true
+  belongs_to :imageable, polymorphic: true, touch: true
 
   validates :title, presence: true
   validate :validate_title_length

app/views/budgets/investments/_investment_show.html.erb

@@ -9,7 +9,6 @@
 
 <% cache [locale_and_user_status(investment),
           investment,
-          investment.image,
           investment.author,
           Flag.flagged?(current_user, investment),
           investment.followed_by?(current_user),

spec/models/proposal_spec.rb

@@ -400,6 +400,14 @@ describe Proposal do
       expect { proposal.update(tag_list: "new tag") }.to change { proposal.cache_version }
     end
 
+    it "expires cache when it has a new image" do
+      expect { create(:image, imageable: proposal) }.to change { proposal.cache_version }
+    end
+
+    it "expires cache when it has a new document" do
+      expect { create(:document, documentable: proposal) }.to change { proposal.cache_version }
+    end
+
     it "expires cache when hidden" do
       expect { proposal.hide }.to change { proposal.cache_version }
     end