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

Closes #2440
[IMP] Website name instead of company name in title and logo
[IMP] mrp: hide some fields on BOM, add groups to handle the displayed fields
[FIX] website: fallback on company name for non-website templates
[FIX] Use website's `domain` field for multi-website

This commit is related to d51157154e478d1fafdd5b7861346535baeed6d2
[IMP] mail: action in notifications.

This commit add the concept of actions linked to notifications. Some subtypes are linked
to server actions. When sending notification to partners, the available actions are
proposed to the user, allowing to perform some actions from the Inbox or from the
notification email.

This is still a WIP.