Enable DiscussionTools as a beta feature on four wikis

arwiki, frwiki, huwiki, nlwiki:
Setting wgDiscussionToolsBeta=true. We must also set
wgDiscussionToolsEnable=true for it to have any effect.

On corresponding beta cluster wikis, it remains enabled
for everyone without a beta feature.

Bug: T245794
Change-Id: I62ae7b8a41d0c5aca23f7f9993964a7f3b84fbba