WatchAction: Clean up redundant getFormFields() and show()

WatchAction: Clean up redundant getFormFields() and show()

* Make FormAction::getFormFields not abstract. In most cases this will just
  be an empty array. This is in prep for RollbackAction as well.

* Remove redundant show() in WatchAction. This used to do custom stuff,
  but after 77cdf1919 it does exactly the same as the parent FormAction::show.

* Don't add 'redirectparams' hidden field if there were no custom query parameters
  (e.g. plain index.php?title=..&action=..)

Change-Id: Ia7f9bb0367c49a23179e9fefa9f529fa8aef8f52