support for interwikis in experimental JS preview

support for interwikis in experimental JS preview

Add support for previewing interwikis in "experimental" JavaScript live
preview.

The #p-lang container is not shown when action=edit by default; it is,
however, shown when action=submit. We check for its existence and if
missing, we add it after #p-tb. (As print/export links are not shown
during either editing or previewing.)

If #p-tb is missing (as the case is in some skins), nothing happens
(and interwikis stay unpreviewable). jQuery is very permissive
and keeps chugging along at all costs.

If #p-lang is missing in the preview page, nothing bad happens as well.
The content of #p-lang is properly cleared and not filled with anything.
(See above comment about jQuery being permissive.)

Change-Id: Idda2e6f7514ba8b1021c719a08855ee9601fa90e