[IMP] mail: parsing emails with several html parts

If an email contains several text/html parts inside a multipart email, the previous code was only keeping the last content part.
The Content-Type: multipart/mixed allows several independent part (RFC1341 7.2.2), so two html is technically valid.
With this patch, the two parts are concatenated. (opw 614755)

Modify append_content_to_html regex to make sure the regex keeps the content of the html instead of removing it.
e.g.: "123 <html> 456 </html> 789" used to be stripped to "123  789" while we expect "123 456 789"
[FIX] res.users: exactly match login when operator in ilike, equal
[FIX] account_invoice: using has_group to determine if user has group group_supplier_inv_check_total

has_group do not care about the fact res.groups is access restricted
[FIX] ir.mail.server: restore parsing of multiple RFC2822 addresses including non-ASCII chars

Rev f2cf6ced1 modified RFC2822 parsing in order to better support
unicode characters inside the Name part of an address header.
However the patch broke handling of multiple addresses (comma
separated) - silently discarding all recipients except the
first one, as soon as any non-ASCII character was present.

This patch restores the functionality while preserving the
fix from f2cf6ced1, and simplifies the code using email.utils
utility functions.

Fixes (again) lp:1272610, OPW 607683
[MERGE] Forward-port 7.0 up to 1b49a87