Removed usage of $wgUser in block and unblock processing:

Removed usage of $wgUser in block and unblock processing:
* Made HTMLFormField pass the HTMLForm object to the validation and filter callbacks (so that they can get a context)
* Added new parameter to SpecialBlock::checkUnblockSelf() to pass the user doing the request
* SpecialBlock::processForm() and SpecialUnblock::processUnblock() now require a context as second parameter; added SpecialBlock::processUIForm() and SpecialUnblock::processUIUnblock() as adaptators from HTMLForm as second parameter to context