Allow the wiki to have private pages somehow

[m@…]

The wiki was brought up publically, we had a few information breaches. It seems the consensus is to be able to have pages be configurably both public or private. No strong decision was made as to which way defaults to, my hat throw is to default-private opt-in public.

Cleeb brought up a MVP in wiki.js that could do this. mediawiki also has plugins for this.

​https://www.mediawiki.org/wiki/Category:Page_specific_user_rights_extensions
​https://www.mediawiki.org/wiki/Security_issues_with_authorization_extensions
​https://www.mediawiki.org/wiki/Manual:Preventing_access#Restrict_viewing_of_all_pages

Related: #123, #6