[FIX] im_chat, im_livechat : fix loading covnersation history and is_in_session function
[FIX] event: avoid double "Publish" button

The event.track view wa sextending the event.event view which was adding action buttons linked to the event.event object instead of the track.
This patch uses the general website view to avoid the confusion (and the risk of publishing an event instead a track).
[FIX] product: Put product_variant_count on template form view.

Was missing to be used in domain added at 92d8f15.
Fixes #2570
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64