Update "comment" notification email to add unsubscribe link

We modified the link that previously redirected us to the "My content"
page to redirect us to the new page for managing subscriptions.

We also adapted the existing generic text by adding a description of
the related notification.

spec/mailers/mailer_spec.rb

@@ -29,4 +29,26 @@ describe Mailer do
       expect(email.subject).to include("commented on your proposal")
     end
   end
+
+  describe "#manage_subscriptions_token" do
+    let(:user) { create(:user) }
+    let(:proposal) { create(:proposal, author: user) }
+    let(:comment) { create(:comment, commentable: proposal) }
+
+    it "generates a subscriptions token when the receiver doesn't have one" do
+      user.update!(subscriptions_token: nil)
+
+      Mailer.comment(comment).deliver_now
+
+      expect(user.reload.subscriptions_token).to be_present
+    end
+
+    it "uses the existing subscriptions token when the receivesr already has one" do
+      user.update!(subscriptions_token: "subscriptions_token_value")
+
+      Mailer.comment(comment).deliver_now
+
+      expect(user.subscriptions_token).to eq "subscriptions_token_value"
+    end
+  end
 end