[IMP] *: one Makefile to rules them all.

Create one unique generic Makefile to compile sass files.
[FIX] web,web_kanban: correct radial-gradient usage
[MERGE] forward port of branch 7.0 up to 3e3e35e
[IMP] point_of_sale: add product description and product sale description to the search engine
[FIX] project: missing translations

The labels for fields 'use_tasks', 'partner_id', 'date_start' had no valid string (autogenerated or unrelated).
Make the selections of the field privacy_visibility translatable.
Fixes #3435
[MERGE] forward port of branch 8.0 up to 87dcaa9
[MERGE] forward port of branch saas-3 up to ec27773
[MERGE] forward port of branch 8.0 up to c825d05
[FIX] doc: guides section was removed, move CDN to reference
[FIX] ir_qweb: css asset: use which to find lessc and sass

Also, use lessc.cmd instead of lessc on Windows. It allows to run
lessc command with only nodejs and the less module (no cygwin).