WIP: allow to define custom prices

This is still a WIP (commit are not clean for example) and the goal is only to make my ongoing changes more visible.

Here is non complete list of important changes:

• one Open Sale Order per Instance Tree instead of one Open Sale Order per user
• create one Hosting Subscription per Open Sale Order line
• Software Instance are not allocated if the Open Sale Order is not created yet. The goal is to use the Sale Trade Condition to setup the Instance Tree related documents security (instead of custom project)
• use the wechat/payzen payment_mode instead of the user language to define the expected payment behaviour
• use the price_currency to define the taxes
• allow defining inherited Sale Trade Condition trees, to define custom price per project/organisation/user/software.
• the Subscription Condition specify which Sale Trade Condition to use
• drop the prices based on service variation. Instead, create a new Sale Trade Condition for a specific software release
• stop copying properties from the Subscription Condition to the Subscription Request

TODO:

• migrating existing Open Sale Order / Hosting Subscription
• specify the expected behaviour of a console based request
• update the rapid space web site if needed
• create custom Service for some software releases
• ...