Change default permissions type from post

Right now even contributors can see and create redirects. This might not be ideal so we should at least restrict it to those with publishing caps or even just to admins.

If we make it easy to change who has access via a filter or just documenting using the roles API then I think restricting to admins is the right call.

No files or tests are named. Start by locating where the plugin defines default permissions for viewing and creating redirects, then review the WordPress roles API and any existing filter hooks. Done means contributors no longer have access by default, while the chosen configurability or documentation is provided.

Written by the indexing model from the issue text.