[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.