[REF] various: clean override of _notify_get_groups

Overrides of _notify_get_groups are used to add some buttons in notification
emails and control the display of "Access document" button. This commit
cleans some overrides :

 * accounting: portal users that are not the customer have no access to the
   invoice and should not have the access button;
 * project: remove unnecessary override on project model and consider portal
   users have access to the document through customer portal;
 * sale: portal users that are not the customer have no access to the sale
   order and should not have the access button;
 * website_blog: display access button to everyone only if the post is
   published; otherwise keep standard behavior;
 * website_forum: display access button to everyone only if the post
   has not been closed or deactivated; otherwise keep standard behavior;
 * website_slides: display access button to everyone only if the slides
   has been published; otherwise keep standard behavior;