Add Page Schemas extension

[yaronkoren]

I never made a push to include Page Schemas in Canasta, even though it's my extension, in part because I remain somewhat disappointed with the way it's turned out - not quite user-friendly or flexible enough to serve as a permanent data structure maintenance tool, as far as I can tell. So I was suprised when I saw recently that, according to the "Used by" template, it's fairly popular, in use by five wiki farms and packages:

https://www.mediawiki.org/wiki/Extension:Page_Schemas

So, maybe it's more useful than I thought - and I think we might as well include it.

[jeffw16]

Sure!