[IMP] website: do not translate sitemap URL
[FIX] ir.translation: export/import of QWeb terms

Fixes the translation term import/export logic to
support terms inside QWeb templates.

Refactored a bit the export code so the babel-based
QWeb terms extractor for ./static/src/*.xml files
uses the same logic as the regular extractor for
ir.ui.views with type QWeb.

Server-side QWeb rendering uses a mix of the native
 view inheritance mechanism and the template inclusion
(t-call) mechanism. During rendering the translations
are only applied at "template" level, *after* the
view inheritance has already been resolved.
As a result translations are local to a template,
not to the inherited view in which they are actually
written.
In terms of exporting PO[T] files, this is done by
resolving the "root" QWeb template a view belongs
to, and using it as the location of the translated term.

During import there is one extra quirk for QWeb
terms: they need to be linked to the `website` model
rather than the actual `ir.ui.view` model they
are really pointing to, so the rendering phase can
properly recognize them.
[FIX] mail, email_template: prevent user from creating
templates in the templat_idm2o field on the wizard; prevent users from
creating models in the model_id m2o field on the template form view.
Merge remote-tracking branch 'upstream/8.0' into 8.0