Make MagicWordArray compatible with PCRE 8.34+

In PCRE 8.34, a subpattern's name must not start with a digit; work around
this by replacing digits in the synonym's index with letters. Amazingly,
this part of the name seems to have the sole purpose of ensuring uniqueness;
none of the matching functions actually use it for anything.

Adding a single-letter prefix was considered, though it would risk breaking
extension code that may have used 29- or 30- character magic word IDs.
(PCRE limits subpattern names to 32 characters.) Likewise, moving the magic
word's ID to the front would not work if it were to start with a digit.

Bug: 58640
Change-Id: Ic69f9000addbf18c4747105187e6f13191828fbb
SECURITY: escape sortKey in pageInfo

DEFAULTSORT isn't escaped before being added to the action=info table.

Bug: 63251
Change-Id: I087bfde8cbc69c3507f68ee3cb6e22aba0ffa7db
Updated release notes and version number for MediaWiki 1.21.9

This is MediaWiki 1.21.9 security and maintenance release

Change-Id: I3c00db1352537170ae36218010c66b9215ddd6c3
Merge branch 'REL1_21' of https://gerrit.wikimedia.org/r/p/mediawiki/core into REL1_21