RemoveOldManualUserPages: Catch errors and enforce bot user group

* Add maintenance script user to the "bot" group to hide it from
  recent changes and avoid triggering certain abuse filters.
* Set $wgUser for other extensions not using RequestContext yet,
  like AbuseFilter.
* Use a username that is in $wgReservedUsernames instead of
  "GlobalCssJs migration script". The edit summary provides enough
  info already.

Change-Id: Ic58c69cdb9d8d888a614ac1cecb7ac7cf039ffde