Handle absurdly long attachment file names

Attachment file names are embedded in the Attachment.content_url field value as a query parameter. For long file names, this was causing the value to exceed the 200 chars default max_length of URLField, and in turn, was breaking event processing. Since this data only ever comes from Zendesk's API, it is "trusted" - for this reason I opted to remove any max_length restriction for the field.