[MERGE] pos improvements

manual fix of fp-style merge
[FIX] tools: html_sanitize: keep mako tags (<% ... %>), required for example in the portal sale quotation email.
[FIX] email_template: when generating body containing the user's signature, add it as html as it is now pure html, not plaintext anymore.
[FIX] mail, BaseModel, portal_sale: fixes and improvements in the URL
management to access documents in notification emails, as well as for the
'view quotation' link in portal_sale module.

models: added a get_access_action method: basically, returns the action to
access a document. It uses the get_formview_action by default (form view
of the document). However for some documents we want to directly go to the
website, leading to an act_url action for some documents. This method allows
this behavior.

portal_sale: get_signup_url now uses the mail.action_mail_redirect method
instead of directly redirecting towards a portal menu. This allows to fall
back on a standard behavior.

portal_sale: get_formview_action updated, to match actions tailored for
portal users.

website_quote: get_access_action of sale order updated. If the sale order
has a template defined, the returned action is an act_url (website view
of the quotation), not the form action anymore.

mail: fixed signature + company signature in notification emails. Even without
user signature, the company signature + access link should be correct.

portal: signup url in notification emali was not using the mail redirection
as action. It is now the case.
[IMP] ir.qweb: avoid repeatedly searching the registry for nonexistant widget models
[IMP] calendar: simplify/optimize `rrule` computed field

Avoid reading one record at a time, leading to multiple
calls to read() which is special-cased and even slower
than a normal one.
[FIX] calendar.read: do not re-compute `display_time` for virtual calendar ID if not requested
[IMP] calendar.event: _compute should be done in batch + consistent @multi attributes
[IMP] orm.Field: doc for @copy attribute
[IMP] hr,mail,product,project,base: use @auto_join=True on _inherits FK field

This can give a performance boost on large databases
and should not be a concern in terms of access control
as the inheritance already grants access to the parent
records.
[IMP] fields.Date[Time]: convert_to_cache: avoid costly parsing when not validating

The extra parsing check is not necessary when we're
not validating inputs, because in that case the
values come from the database and are valid.
The validation is quite expensive due to calls
to strptime() + strftime().
[FIX] ir_actions in website: allow to compute website_path even when creating a new server action (avoid crash then)