[account_mtd_vat] Determine whether box 5 was originally negative from GUI in view vat return form

[Nick-OpusVL]

This is to allows us to display what HMRC give us back unchanged while still giving accountants info they need about whether we owe or HMRC owe us.

A non-stored computed field {'positive', 'negative', 'zero'} net_due_sign {'reclaim', 'payment', 'nothing'} (name TBD) - dependent on whether we owe HMRC or they owe us or neither.

Get clarification from Stuart or someone in accounts as to which of 'positive' or 'negative' to use in which situation.

Another non-stored computed display field based on that to show 'Payment', 'Reclaim' or '' based on computed state.

[Nick-OpusVL]

Actually I suggest we go straight to a status field of 'reclaim', 'payment' or 'nothing', and name it appropriately, rather than using 'positive', 'negative' or 'zero'. The former have much more semantic meaning for this purpose than +ve, -ve or 0 (in which case someone needs to remember which it is).

I will still compute another field for the display string off that so that the decision and display logic are kept separate.