Special:SimilarEditors should only be accessible by checkusers, as specified in T304520.

We currently require that a user has the 'checkuser' right in order to access Special:SimilarEditors. However, this created a dependency on the CheckUser extension.

Instead, we should introduce a new right, similareditors, and assign this to the checkuser group in the Wikimedia production config.

We will need to:

• Add the similareditors right to extension.json's AvailableRights
• Pass in similareditors (instead of checkuser) into the parent constructor, from SpecialSimilarEditors::__construct