[IMP] mail: notification emails with button

Notification emails have been redesigned. They notably include buttons
allowing to perform some action directly from the email.

The notification creation and sending has been partially rewritten
and improved. The purpose is to lessen the number of rendering to perform
when sending emails to recipients. Recipients are first categorized into
groups. Basic groups are partners and users. The notification template
is then rendered twice, one for followers and one for not-followers. In most
cases there will be few rendering to perform. Through inheritance it
is possible to further categorize users. For example HR users / officers
that have approve / refuse buttons in their email.

A custom data structure is used to store data about buttons and actions.
URLs, follow / unfollow are added in the structure and used in the
template to render the email for a given group.

New routes are added in mail. Those allow to perform some action, like
going to a form in create mode, following / unfollowing, executing a method,
sending a signal for a workflow. Those routes are for users only and rely
on classic access rights.

A generic route for viewing records is added. It replaces the old redirect
action. According to some specific action given by the already-existing
get_access_action, the record will be visible for everybody (forum, blog)
or restricted (going on the Inbox / login / form view, according to access
rights).

The next commit will add the various inherits necessary to add the actions
in the main addons.