`@groups` attribute in qweb views

Handling of `@groups` in qweb views was added at some point, but the interaction with edition (RTE) is completely broken:

* If the view is edited by a user who does not have the groups, the element is removed and the view is then saved with the element removed
* If the view is edited by a user who *does* have the groups, the attribute is removed from the element when saving the view, making it visible to everybody

For now, fixed by rendering `@groups` to the DOM in the latter case, and by disabling the RTE and snippets (with a help tooltip) in the former one.

If case 1 is too annoying in the long run, it might be possible to restrict the uneditability only to the branded section holding the `@group`, rather than to the whole top-level page.