Since the deletion of `web_tip` module (6416c46d8504573eacbc49d7ca798c3d8a53fd44),
this link module is useless.

The code in mail_tip included former implementation of the timeline view, which
is now replaced by slack.

This commit introduces a new view type, the timeline view. This view is
intended to display messages like the previous chatter. This is now a view
like the form or list view. Like other views it will be possible to have
custom templates for some specific needs, allowing customization.

[IMP] mail: the value tracking is modified. Previously messages were created
containing the modified values. Those values are now stored, using a new
model mail.tracking.value. The message body is dynamically build based on
the values. Tests have been updated.

This version is temporary. Indeed two main modifications will come in a short
future :

 - the new design will improve the display
 - the slack mode will change the way the Inbox and notifications are managed

All glory to the Hypnotoad.

Special thanks to Valerie Pirenne (vpi), Jerome Maes (jem) and Richat Mathot
(rim) that did not code but said a lot of things. Martin Trigaux (mat) did
nothing, a bit like for the slides modules, but he is busy sending emails.

The module system needs to know the dependencies of a given module
before executing the function.  This is why the dependencies were
defined once in an array, and then were described one more times in the
call to require.

But a trick can simplify this: the boot function can parse the string
representation of the module and extract the calls to require from it.
It is more work for the processor, but it leads to simpler module
definitions.

* Override message_fetch from mail.Thread in a dedicated limk module (mail_tip)
and fire an event when the messages are loaded. This is necessary for tips on
the chatter.
* Override render_value form web.form.FieldStatus: Call reposition() when a
FieldStatus object is done rendering.
* Tip.reposition() method
* Handle web bus resize event