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.
