Make MySQLi work with non-standard socket

MySQLi needs to specify the socket location as a parameter, and not as part
of the hostname.

Split the socket out of the hostname if it contains *one* colon (IPv6 safe)
after checking for the port number.

Conflicts:
	includes/db/DatabaseMysqli.php

Bug: 65998
Change-Id: I66ee34e1029bc1630669bde4272d0eae9d3fd9f1
GlobalVarConfig shouldn't throw exceptions for null-valued config settings

Use array_key_exists rather than isset to test $GLOBALS.

Bug: 66986
Change-Id: I9beabd79e1d52966b9184c9647af5a2a31c19276
(cherry picked from commit 1a059dbbe2380e9ae752daf6c5583333b5bd5f07)
SECURITY: Enhance CSS filtering in SVG files

* Filter <style> elements
* Normalize style elements and attributes before filtering
* Add checks for attributes that contain css
* Add unit tests for html5sec and reported bugs

Bug:69008
Change-Id: I732eece710f1bfaaeea1e5de541fcd4cfb375de7
Updated release notes and version number for MediaWiki 1.23.4

This is MediaWiki 1.23.4 security and maintenance release.

Change-Id: I62067c60794a002e012dc6f8c01b2a2f9fb3ed82
Merge branch 'REL1_23' of https://gerrit.wikimedia.org/r/p/mediawiki/core into REL1_23