Requests for comment/Shadow namespaces

A lot of this depends on the implementation details of global scripts (cf. 39610). However, in either implementation, it would be possible (and advantageous) to have a central repository somewhere.

Scope
For the purposes of this RFC, we're looking at least:


 * Global gadgets
 * Global Scribunto modules
 * Global templates???

commons.wikimedia.org
Outside project scope? Already treated as a central repo with precedence support with File links (i.e., will use the Commons version, unless a local version exists).

wikidata.org
Outside project scope? Already doesn't really exist right now.

meta.wikimedia.org
Global shit already largely lives here.

scripts.wikimedia.org
Hmmmm.

scripts.mediawiki.org
Are we serving Wikimedia wikis or any MediaWiki user? This could always be a redirect as well, of course.