Wikimedia wikis configuration for $wgExtraSignatureNamespaces

TL;DR: no-op, but reinforces current config in a way that will be
useful to VisualEditor as well (bug 51154).

Newly introduced configuration variable (in I7ccf1093), defines some
additional namespaces where the toolbar is expected to have a button
to insert signatures, in addition to the default set of namespaces
where this is the case.

The current MediaWiki default is to have a button on all namespaces,
while I7ccf1093 will remove it from content namespaces, i.e. usually
only namespace 0. However, on most Wikimedia projects there are
important discussion pages on NS_PROJECT (see e.g. village pumps on
[[m:Distribution list]]) as well as NS_HELP (see e.g. help desks at
[[wikidata:Q4026300]]), so for additional safety we specify those
redundantly here to avoid depending on core. For those wikis, this
changes nothing.

Additionally, most non-content wikis have a looser content separation,
closer to the UseModWiki time: most notably Meta-Wiki, but also the
other "special" and "wikimedia" (chapter) wikis, which often have
discussions, including their "village pump" equivalent, in ns 0. There
is no gain in disabling signatures in any of those, just keep it
enabled.

Cherry picked from commit 57aa7a551db10cbab5aa3aa9e70a0661f19b0fc4

Co-Authored-By: MatmaRex <matma.rex@gmail.com>
Change-Id: I77b6a3dc3df35711180c99e2ff05cfe650f9fdbc