[FIX] website: translator: actually re-translate when asked for

If you want to retranslate an item, the website translator was creating
a new ir.translation record because it is not able to know if the
item you're working on is already translated or not. There seems
to be an unfinissed work in progress about this (see oeTranslationId
and translation_id).

This commit adds a 'translated' attribute when ir.ui.view translates
the qweb view and uses it during the website's "set_translations"
method. If translated, then look for a matching ir.translation record
and udpate it.

It'll probably be more effective to really store the translation_id
during the qweb view's translation.