Message are now fetched and display using javascript
to optimize performance.
Also the chatter is completely rewrite : add pager,
filter possiblity, ...

This commit is mainly technical: posting feature does
not change. Only the display with pager is new.
The goal is also to prepare frontend chatter for a
better integration with rating widget.

The mail.thread mixin is extended to manage the field
website_message_ids (display all messages that can be
seen on frontend by user (employee or not).

When fetching or posting message, a check is done to see
if we need to do it as sudo(). website_mail implement posting
messages with token or sha_in (using in website_quote).
The same verification is now done when fetching messages, via
'_special_access_object' method.

A new generic chatter template is available in website_mail
This template allows access rights escalation when some kind of token or uuid
is available on the model or if you use the object_shasign function in the
main controller of website_maill to generate a cryptographic signature to allow
commenting on any object.

To use this chatter, you need to make a t-call to website_maill.thread in your
template after having set the following variables:
- chatter_object: the browserecord of the mail_thread object (mandatory)
- token: if you use a token system  (optional)
- token_field: name of the field that stores the token on your object  (optional)
- sha_in: if you use a shasign to allow public comment  (optional)
- nosubscribe: set False if you want the partner to be set as follower of the object  (optional)
- message_type, subtype: see message_post in mail_thread.py