Combine JavaScript and JSON encoding logic

This will help with improving human readability of JS and JSON
objects encoded by both ResourceLoader and the API. This patch
also adds new "utf8" parameter to the JSON formatter of the API.

Changes to FormatJson class:

* Added escaping of '<', '>', and '&' by default to protect against XSS.
* Removed unnecessary escaping of '/' and added an additional option to
  unescape non-ASCII characters (those above U+007F) as well.
* Added PHP 5.3 pretty printing code (to replace Services_JSON) that
  uses a four-space indent as PHP 5.4 does.

Changes to Xml class:

* Defined Xml::encodeJsVar() in terms of FormatJson::encode()
  and added a pretty printing option. Also added a pretty printing
  option to Xml::encodeJsCall() as well.
* Deprecated Xml::escapeJsString() and QuickTemplate::jstext();
  callers have to add quotes themselves, hence the escaping of
  both double quotes and apostrophes.

(cherry picked from commit 79f80cc49571e286b89fc1666c034262611fac7f)

Conflicts:
	RELEASE-NOTES-1.21
	tests/phpunit/includes/json/ServicesJsonTest.php

Bug: 26818
Change-Id: I1987190f1ba5bf41738e7bd611209706c1f6bb5c