* Use the context when building the preferences form descriptor

* Use the context when building the preferences form descriptor
* Pass the HTMLForm object to the submit callback so that it can have a context and also access to the form itself (see below). This makes Preferences::tryUISubmit() and Prefrences::tryFormSubmit() signatures incompatible. The only extension calling these is EditUser and I will fix in my next commit.
* Added two extension possibilites (for EditUser): PreferencesForm::getExtraSuccessRedirectParameters() to modify the URL parameters when redirecting after successful save and the fourthparameter to Prefrences::getFormObject() to be able to remove some items from the descriptor
* Pass the user being modified to the HTMLForm to use it to save the preferences instead of doing this unconditionally on $wgUser (to remove one hack in EditUser preferences)