[IMP] product: remove UoM constraint from product module and give it in stock and account module

Closes #2440
[IMP] Event: Split Tickets
Attendees are created on SO confirm.
Attendees are directly created for FREE registrations and
Attendees are created and SO are linked to the paid tickets.
Improved code to make (5 attendees lines, 2 SO lines) when 3 vip tickets and 2 standard tickets are bought.

This commit fullfills the following Goals:

Goal 1 : When only website_event is installed, free registration should be possible from website.
Goal 2 : When only event and website_event are installed, badge report should be print for free registrations.
Goal 3 : When website_event_sale is installed, at that time on confirming the sale order, the subscription line are splited per attendees.
Goal 4 : Added code for attendees registrations through SO. Now from SO also the attendee's details for the event can be filled same as from website