Add and apply FileRead and FileWrite rubocop rules

They were added in Rubocop 1.24.0.

Even if we were already applying FileRead everywhere, this is something
we've manually fixed in the past. Another reason to add it is that these
rules are deeply related.

.rubocop.yml

@@ -485,6 +485,12 @@ Style/ClassVars:
 Style/CollectionMethods:
   Enabled: true
 
+Style/FileRead:
+  Enabled: true
+
+Style/FileWrite:
+  Enabled: true
+
 Style/HashConversion:
   Enabled: true
 

app/models/concerns/json_exporter.rb

@@ -2,7 +2,7 @@ module JsonExporter
   def to_json_file(filename)
     data = []
     model.find_each { |record| data << json_values(record) }
-    File.open(filename, "w") { |file| file.write(data.to_json) }
+    File.write(filename, data.to_json)
   end
 
   private

spec/system/admin/machine_learning_spec.rb

@@ -216,9 +216,9 @@ describe "Machine learning" do
              kind: "comments_summary",
              updated_at: 2.minutes.from_now)
       comments_file = MachineLearning::DATA_FOLDER.join(MachineLearning.comments_filename)
-      File.open(comments_file, "w") { |file| file.write([].to_json) }
+      File.write(comments_file, [].to_json)
       proposals_comments_summary_file = MachineLearning::DATA_FOLDER.join(MachineLearning.proposals_comments_summary_filename)
-      File.open(proposals_comments_summary_file, "w") { |file| file.write([].to_json) }
+      File.write(proposals_comments_summary_file, [].to_json)
     end
 
     visit admin_machine_learning_path