[FIX] event: translate templates into user language

When sending an email of the registration/confirmation, the context was lost and the tempate sticked in en_US.
With the patch, the template will use the current user's language.
[IMP] stock: display translated selection value in message

When a user tried to delete a done or canceled picking, the error messages used to display the key of the selection field ('done' or 'cancel') which was surprising in other languages than English. This patch takes the string value of the selection field, keeping the context to get the translated value (opw 613068)
[FIX] purchase: keep fallback behaviour that was removed at rev 8973217

If no valid invocie is found on the purchase order line, use the price and currency of the line (coherent with help message)
Forward port of 7.0 up to rev 9b87d6f
[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
[FIX] correctly forward context in graph view

the new graph view silently ignored the context when doing its rpc
read_group.  Usually, it's not really a problem, which is why it is only
now being fixed, but some models actually use the context in read_group.
(for ex, account_entries_report)
[FIX] Previous move subtleties
[IMP] Assign all destination moves at once, add packaging
Merge pull request #2603 from odoo-dev/8.0-wmsstaging10-jco

8.0 wmsstaging10 jco
[FIX] account_voucher: convertion rate of purchase receipt

The conversion of currencies in vouchers are made based on payment_rate_currency_id field (instead of usual currency_id).
This field was not present in the purchase view and not correctly set when we change the journal (and thus the currency) which was leading to rates of 1. With a rate of 1, the amount is equal to the currency amount (which is rarely correct). opw 611254
[FIX] website_twitter: display setup/debug info only for website editors, not visitors
[FIX] auth_ldap: remove unused sql column
[FIX] account_voucher: import invoices, sign according to transaction type
[IMP] crm: make code of merge opportunities overloadable
[FIX] website_blog: portal users should not be able to read unpublished blog posts
[FIX] models: only check existance of inverse_field for one2many fields
[IMP] mail: avoid sending an email twice

In some specific conditions (e.g. admin logging during cron run, concurrent update), writing on the mail object may fail, triggering a rollback of the transaction.
As the write was done after the SMTP sendmail action, an email could be sent twice (state not correctly put, would be retried at next process of the email queue).
This patch provoke the error before the sendmail action and thus avoids sending the email.
Fixes #552
[IMP] base: safer locking at user login

When a users connects, a lock is taken on the res_user table to modify the last login date. If another running transaction uses a foreign key to res.users (e.g. write_uid column), postgres may detect the update as a concurrent update and rollback the transaction.
In pg 9.3, the lock_strength parameter 'NO KEY' allows a weaker lock which is less likely to break another transaction.
Fixes #552