[FIX] website_forum: fix broken forum activation email due to the migration to the new API.

Next step: write tests. Hooray.
[FIX] hr_holidays: fix virtual remaining leaves computation.

Now only validated allocation requests are taken into account
when computing the number of virtual remaining leaves (leaves
+ pending leaves). This avoid creating allocation request to
have virtual remaining leaves artificially added.
[IMP] mass_mailing: add creation date in fields
to display it on list view + perform search on it.
Merge remote-tracking branch 'upstream/master'