Widgets and website display images or display the type with icon like pdf, doc...

Replace deprecate controllers like /web/binary/image, /web/binary/saveas...
Use ETag for all content with 'unique' option to cache the content if the content is never changed.

Main changes :
- mail.notification model is removed. People do not receive notifications
anymore. Instead two ways of following documents exist
  - using a channel; messages will be displayed on the channel itself
  in a near future commit
  - following with its partner; messages will all be considered as
  needaction, using a new m2m table. People should receive less
  needaction messages by following less records by themselves.
There is no more read / unread state anymore. Instead only needaction
messages are considered. Todo (Favorites) messages still exist, and are
stores on a new m2m table instead of using decorated notifciations.
- the main filter for documents is not message_unread anymore, but
message_needaction. A lot of views and filters have been updated
accordingly.
- the vote feature has been removed

Removed fields and code related to the display of groups and employee
suggestions. This impacts notably the model of res.users in mail and
hr.

When the attachment type is not defined, the icon used must be unknown.png

closes#7336

Starting from now, subtypes can be internal. This means that only employees
(group_user members) can see the messages with this subtype. This feature
replaces and enhances the 'log a note = no subtype = internal only'
feature.

Public and portal users cannot see internal messages. This allows to have
subtypes and a follow mechanism that works for employees and is not
visible for external people.

Its first use will be for Crm Activities, allowing to have custom
subtypes visible only for salesman and not send to the customer.