[ADD] ability to trigger retrying rendering with an updated rendering context
[IMP] no need to compute *all* has_group calls if the first one fails
[IMP] prevent edition of page if groups which current user doesn't have

If the current view uses @groups attributes (possibly in called templates),
the corresponding elements are rendered to a void (empty string in qweb). If
said user can edit the page, does so and saves a view section in which there's
a @groups to which he has no access, the element[@groups] is completely
removed from the template once saved, losing it.

If QWeb encounters an @groups to which the current user has no right during
rendering, have it request a no-RTE page, so the user can not RTE-edit the
page (or drop snippets in it).
[FIX] @groups attribute should not disappear when editing

The @groups attribute in qweb views was not rendered (even when matched by the
user), so editing a template with an @groups would either remove the whole
section (if the user didn't have the groups, fixed in previous commit) or only
removed the attribute itself making it visible to everybody (which ought be
fixed-ish by this commit).
[FIX] report rendering should ignore http.Retry
Merge pull request #584 from xmo-odoo/master-qweb-groups-xmo

`@groups` attribute in qweb views