Group cache reset does not work in Firefox

In firefox pressing the "Group Cache reset" button does nothing.

This seems to happen because the from us submitted as enctype="multipart/form-data" (likely to avoid that the action_resetgroupcache parameter is interpreted as an action). However it seems that FF does not send this parameter at all.

A simpler way is to rename the parameter to e.g. resetgroupcache_action which makes the enctype unnecessary.