Disable ID based partial caching for all security actions

Disables ID based partial caching for all security actions so that actions such as Security/lostpassword and Security/passwordsent work properly even if partial caching is used.
FIX allow children to be accessed via template
Merge pull request #1955 from willmorgan/patch-2

FIX allow children to be accessed via template
API Remove AjaxUniqueTextField, since its operation is very limited (#1947)

It used to validate via ajax, but that has been removed as part
of the 3.0 refactor of all prototype.js code. In the end,
its a field which is quite trivial to implement in custom code
(or in controller code), unless we re-add the Ajax features.
And I think an ajax-validating uniqueness field is too much
of an edge case to belong into core.
Merge branch '3.1'

Conflicts:
	README.md
	dev/install/install.php5
	forms/ConfirmedPasswordField.php
	tests/forms/FormTest.php
Merge pull request #1974 from chillu/pulls/ajaxunique-remove

API Remove AjaxUniqueTextField, since its operation is very limited (#19...
Merge pull request #1780 from hdrlab/patch-4

Disable ID based partial caching for all security actions
FEATURE Add Convert::raw2htmlid()
Remove references to ComplexTableField
PHPDoc updates
PHPDoc
API: Form Field ID attribute should follow HTML specification

Fixes: http://open.silverstripe.org/ticket/4431.

Changes Form and Form Field classes to make use of Convert::raw2htmlid() which follows http://www.w3.org/TR/REC-html40/types.html#type-cdata.

Introduces a FormTemplateHelper class to assist in these sort of updates in the future.
FIX: Update references to ID values from 79c9433