Improve interface when using custom capabilities on single installation

[JoryHogeveen]

Copied from: #7218

When it's not a multisite but a single installation site admins do not automatically get capabilities assigned.

We could perhaps do a check when they set a cap there to warn them that their role doesn’t have that cap yet, but I don’t think there’s anything we could do there besides possibly some workflow improvements like:

1. Notice: we could show a notice next load of the pod edit screen for a CPT/CT when cap is not set on the current user role
2. Notice: we could add HTML field that shows a notice below the cap setting that shows only if the value of that cap is not in the list of the current user role’s caps
3. Action: we could add checkbox field that shows a notice below the cap setting that shows only if the value of that cap is not in the list of the current user role’s caps and allow them to check it to auto-add the cap to the current user role