Updates the CMS profile page and SecurityAdmin to give developers a few ways to customise the required fields.
Added extension hook updateValidator for getValidator for things like modules to inject required fields to go along with Injector for replacing the entire class for project specific use.
See discussion at https://groups.google.com/forum/?fromgroups#!topic/silverstripe-dev/Dodomh9QZjk
Fixes an access issue where all public methods on FormField were allowed,
and not checked for $allowed_actions. Before this patch you could e.g.
call FormField->Value() on the first field by using action_Value.
Removes the following assertion because it only worked due to RequestHandlingTest_AllowedControllerExtension
*not* having $allowed_extensions declared: "Actions on magic methods are only accessible if explicitly allowed on the controller."
