Fields for page-specific permissions are inconsistently named

[timobrembeck]

Motivation

The fields for page-specific permissions of the Page model:

• The editors field contains all users who have the permission to edit (but not publish) the page: https://github.com/digitalfabrik/integreat-cms/blob/d68c72f9274ec0d7691dc914250194ac062691a1/integreat_cms/cms/models/pages/page.py#L159-L170
• The publishers field contains all users who have the permission to edit and publish the page: https://github.com/digitalfabrik/integreat-cms/blob/d68c72f9274ec0d7691dc914250194ac062691a1/integreat_cms/cms/models/pages/page.py#L171-L182

Proposed Solution

Since our roles are called "Author" for the users who have the permission to edit (but not publish) a page and "Editor" for the users who have the permission to edit & publish the page, I suggest to rename

• editors to authors and
• publishers to editors.

Additional Context

Make sure that the data migration preserves existing users in these fields (so the renaming should not lead to privilege escalations of the users specified in the editors field).