Support user relationships besides follow (such as Friend)

Before Commons 3.0, commons supported a user relationship feature. Relationships could be symmetrical or asymmetrical ... by default a "Friend" relationship was configured and available, but site administrators could configure there own relationships such as "mentor", etc.. This is a feature that didn't make it to the MVP for Commons 3.0. This should either be added back into Commons or documented as deprecated.

Meta issue for 6.x-2.x features that didn't make it to Commons 3.0 MVP: #1930406: [Meta] - As a site Administrator I would like to upgrade from 6.x-2.x to 7.x-3.? without losing functionality.

Previously this was done via the user relationships module.