Enable better role/capability management

[eteubert]

The plugin uses the "administrator" capability in most pages. Some sites may want to provide access to editors without making them admins.

• [ ] figure out what's the best way to achieve this: a dedicated role/capability? what are the options? Please discuss / suggest!
• [ ] implement

via https://wordpress.org/support/topic/create-new-user-role-like-podlove-editor/