SECURITY: Check SVG xml encoding against whitelist

Some browsers will interpret obscure xml encodings as UTF-8, while
PHP/expat will interpret the given encoding in the xml declaration.

bug: 47304
Change-Id: I3b311a7078d977ae89c51e95e625d79fba183cfc
Add hook AbortChangePassword to Special:ChangePassword

Adds a consistent method for authentication extensions to block
password changing operations.

Conflicts:
	includes/specials/SpecialChangePassword.php

Bug: 46590
Change-Id: I3469e90a958c4fb0f24cafd67de5590d3cc2f075