[IMP] doc/howtos/backend: add a small section on how to start the server
[FIX] mail: do not send email at module installation
[FIX] point_of_sale: the category list now uses less space when the categories don't have pictures, freeing some screen space on tablets
[FIX] model: prevent exporting column ID of non ordinary tables

This makes no sense and can generate errors if you try to purge
ir.model.data of not existing records.