Simplify DiscussionTools config

We had wgDiscussionToolsBeta=true set individually for every wiki.
Just make it the default.

Moved the comments documenting why the extension is enabled on each
wiki to the wgDiscussionToolsEnable section.

This change should be a no-op.

Change-Id: I5604ac6f00e23b895592a0dc2232d2e16b7726c8