Remove notion of editors

[leplatrem]

The editors group is used to limit the users allowed to request review on a collection.

We don't really need that. Being able to write on the collection is enough to be allowed to request review. And there is no reason why a reviewer could not request a review too (since they have write perms on the collection).

This would allow to remove a lot of duplicated lists in our permissions manifests, because for collections whose group of editors is the same as reviewers, we would just have:

cid:
  permissions:
      write:
          - /buckets/main/groups/cid-reviewers

And for the collections which have editors that are not reviewers, we would do:

cid:
   permissions:
      write:
          - ldap:editor1@mozilla.com
          - ldap:editor2@mozilla.com
          - /buckets/main/groups/cid-reviewers

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This issue was initially reported here Kinto/kinto-signer#733

But it makes more sense to keep track of it here, along with #1945, #1946, #1948.

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Imported from mozilla-services/kinto-dist#1952