add "Reason" field to the the discarded pages

UI experiecnce, something like :

[discard, blank extra sheet]
[discard, blank other]
[discard, scrap paper on extra sheet]
[discard, scrap paper other]
[discard, duplicate]
[discard, any other reason], specify [..........]

The future duplicate resolver could also fill in this field when doing collision resolution.

But first steps: add to DB.