Migrate 'Attachment' to be an issue field

Moving 'Attachment' to be an actual issue field provides users with greate control over the field in issue creation - e.g. adding a further description, hiding the field and re-ordering the field.

However, 'Attachment' should be hideable but not requireable and not required - always optional.

When editing an issue, if 'Attachment' was set as required, this would mean that an attachment would need to be added each time.

This does not fit with the current issue field design - which enforces a field to be either requireable or required.