Contextual Personas on top of Sitewide Roles

* As described in #610, User Roles are personas and collections of capabilities.
* User gets capabilities at runtime
* from Sitewide Roles stored in `wp_options`
* from Personal capabilities stored in `wp_usermeta`

This can be extended to create Object specific roles (called Personas) which would be a collection of capabilities just like Roles stored in `wp_postmeta`.

This can be further extended to provide personal capabilities specific to an object stored in `wp_llms_user_postmeta`.

At any given point during the course experience or administration, the users' total capabilities would be a result of all the above. This would allow very granular and simple permission and privacy management at every level. This would make it easy to create extremely flexible and drippable permission and privacy module in LifterLMS for courses and memberships.