Add self relation concept to model definition

[a0js]

Suppose I have the following model, representing the permission for a person to update their own notification preferences or an admin updating those permissions:

model
  schema 1.1

type person
  relations
    org: [organization]
    me: [person]
    can_update_notification_preferences: me or admin from org

type organization
  relations
    admin: [person]

In order for this to work in the current system. We have to have a me relation on the person, and a corresponding self relating tuple (person:1, me, person:1) in the database.

It would be nice if we could add a keyword (self, this, etc.) to reference the object itself as a potential user on a relation. For example:

type person
  relations
    org: [organization]
    can_update_notification_preferences: self or admin from org

OR

type person
  relations
    org: [organization]
    can_update_notification_preferences: [self] or admin from org

OR

type person
  relations
    org: [organization]
    can_update_notification_preferences: [person#this] or admin from org

I'm not sure what the correct keyword would be or what the easiest implementation would be, but it would be a helpful relation definition to have without the extra tuples and relation.

Related to #259