[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.
[FIX] mail: when having only mail installed, do not show any 'share with my followers' compose box. This comes only with hr, for the inbox. This was probably forgotten when updating the mailboxes hr-goal and hr-related gamification / chatter stuff.
[FIX] mail: posting on the user should be redirect to its partner, not nowhere.