[FIX] account: recompute date_due

Add on_change to recompute the due date when the invoice date is modified (it was only done when modified the partner_id field
Fixes #3413
[FIX] stock_picking_wave: Missing _description
[FIX] Packaging: Debian: ignore root Makefile

Commit 388732dfa6a1 introduced a makefile at the root of the project, and
dh_auto_build is using this file instead of setup.py. Explicitely tell
debhelper to not use the makefile.
[FIX] point_of_sale: when loading models with a specified id, use read instead of search_read, to prevent inactive records from being skipped
[FIX] /web/binary/{image,saveas}: use request.registry instead of deprecated `request.session.model`

This makes the controllers work also for
public users, e.g. in the frontend.
[FIX] website_forum: avoid access error when closing spam/abuse questions

Logging the user login is not necessary, but
accessing it might cause access errors. Logging
the UID is equivalent anyway, so let's not bother.
Merge branch '8.0' into 8.0-pos-backports-fva
[IMP] point_of_sale: add helper function to compute tax-specific subtotals, useful for receipt customization