Summary
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Mandrill Integration cube

provides 2 main features :

1. Hijack the existing functions that call config.sendmails

    In these functions, a "sendmails" service is called instead of
    `config.sendmails`, this service being provided by the mandrill cube.

    https://www.cubicweb.org/ticket/3797393 proposes to replace
    config.sendmails by a service in the core.

2. Provide an improved NotificationView

    Mandrill provides a template api :
    https://mandrillapp.com/api/docs/messages.python.html#method=send-template

    The MandrillNotificationViewMixin makes use of it and allows to send
    different emails to each recipient.

    Beside using the send-template api of mandrill, it gives more control to
    the developer on the sender, and the merge variables for each recipient.

    Since it is a Mixin class, it can be mixed in an existing notification to
    make it instantly usable with a mandrill template.
