[FIX] base: almost-duplicated translations ignored during import

Translations are not transfered from temporary table tmp_ir_translation_import
to ir_translation if translation already exists (using `find_expr`).
In case of import of QWeb terms (name = 'website'), the criteria was too weak,
finding already present terms in translations for different modules.
e.g. term "Quantity" is already present in a QWeb view from sale module and was
not imported for the translations of the website_sale module.

Fixes #4239