POS modifications

Numerous modifications of the Point Of Sale :
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- Customer pannel
- Editing / adding customers / link to sponsor
- Intuitive browsing for customers (right pan) - by letter
- Browse customer sales history : see its sales and all its products
- Cashier pannel at bottom-left
- Custom packs (can contain templates and their variants : on demand)
- POS internal messaging (instant, delayed, recurrent / text or image)
- Special discount : shop manager can allow special discount by entering
a password
Revert "Add Partner Firstname"

This reverts commit 35c060297ed0a9178454b23e2d606a7064caffa7.