Add how to pay on invoice payment

More elegant duck

* Add first draft of "my invoices" template
* Some changes on links name

Change CoinLdapSyncModel to a Mixin class because it's more appropriate (This just add/modify method to somes models, it's more a role than a type of object)

Revert "[fix] Informations du membre: Gestion du cas où un utilisateur n'est pas rattaché à un membre"

This reverts commit 1d930a99.

This commit inadvertently removed the change_password() function.

It's still not perfect, see comment in the code.