Add helper text to datetime-local form fields

Currently, datetime form fields are ambiguous because it's unclear whether
the application timezone or system timezone will be used. This commit
adds helper text to datetime form fields specifying that the system
timezone will be used for datetime-local parameter inputs.

app/helpers/maintenance_tasks/tasks_helper.rb

@@ -109,7 +109,7 @@ def parameter_field(form_builder, parameter_name)
       when ActiveModel::Type::Decimal, ActiveModel::Type::Float
         form_builder.number_field(parameter_name, { step: "any" })
       when ActiveModel::Type::DateTime
-        form_builder.datetime_field(parameter_name)
+        form_builder.datetime_field(parameter_name) + datetime_field_help_text
       when ActiveModel::Type::Date
         form_builder.date_field(parameter_name)
       when ActiveModel::Type::Time
@@ -120,5 +120,13 @@ def parameter_field(form_builder, parameter_name)
         form_builder.text_area(parameter_name, class: "textarea")
       end
     end
+
+    # Return helper text for the datetime-local form field.
+    def datetime_field_help_text
+      tag.div(
+        tag.p("Datetime inputs will use the system timezone, which is #{Time.now.zone}."),
+        class: "content is-small",
+      )
+    end
   end
 end