DEV: Set limits on custom fields

This patch sets some limits on custom fields:
- an entity can’t have more than 100 custom fields defined on it
- a custom field can’t hold a value greater than 10,000,000 characters

The current implementation of custom fields is relatively complex and
does an upsert in SQL at some point, thus preventing to simply add an
`ActiveRecord` validation on the custom field model without having to
rewrite a part of the existing logic.
That’s one of the reasons this patch is implementing validations in the
`HasCustomField` module adding them to the model including the module.

app/models/concerns/has_custom_fields.rb

@@ -60,12 +60,19 @@ module HasCustomFields
     end
   end
 
-  included do
-    has_many :_custom_fields, dependent: :destroy, class_name: "#{name}CustomField"
-    after_save :save_custom_fields
+  CUSTOM_FIELDS_MAX_ITEMS = 100
+  CUSTOM_FIELDS_MAX_VALUE_LENGTH = 10_000_000
 
+  included do
     attr_reader :preloaded_custom_fields
 
+    has_many :_custom_fields, dependent: :destroy, class_name: "#{name}CustomField"
+
+    validate :custom_fields_max_items, unless: :custom_fields_clean?
+    validate :custom_fields_value_length, unless: :custom_fields_clean?
+
+    after_save :save_custom_fields
+
     def custom_fields_fk
       @custom_fields_fk ||= "#{_custom_fields.reflect_on_all_associations(:belongs_to)[0].name}_id"
     end
@@ -133,6 +140,28 @@ module HasCustomFields
           end
       end
     end
+
+    private
+
+    def custom_fields_max_items
+      if custom_fields.size > CUSTOM_FIELDS_MAX_ITEMS
+        errors.add(
+          :base,
+          I18n.t("custom_fields.validations.max_items", max_items_number: CUSTOM_FIELDS_MAX_ITEMS),
+        )
+      end
+    end
+
+    def custom_fields_value_length
+      return if custom_fields.values.all? { _1.to_s.size <= CUSTOM_FIELDS_MAX_VALUE_LENGTH }
+      errors.add(
+        :base,
+        I18n.t(
+          "custom_fields.validations.max_value_length",
+          max_value_length: CUSTOM_FIELDS_MAX_VALUE_LENGTH,
+        ),
+      )
+    end
   end
 
   def reload(options = nil)