[WikiEN-l] No-indexing of project-space pages

[Simetrical]

I've added __INDEX__ and __NOINDEX__ magic words in r37973.  A
configuration option might be added that would optionally disable
them, or I might be reverted for other reasons, but the capability for
user-added per-page indexing changes has now been written, in any
event.

As I implemented it, any user can add or remove the indexing keywords.
 This is necessary given that they're implemented as magic words and
not a special page or whatever.  I did it that way because Brion
seemed to favor it in his comments on bugs 8068 and 9415.

This would allow namespaces to be no-indexed by default (with a config
change), with __INDEX__ used to re-index individual pages, like policy
and essay pages in the Wikipedia namespace but not procedural pages
like DRV or RFA.