Provide developer debug permissions

Problem/Motivation

In some cases we want to expose a bit more information to developers so
that they can quickly diagnose issues. Information that may not be
understandable to site managers (such as raw field names).

Steps to reproduce

Proposed resolution

To provide a unified solution for this that does not rely on the
developer user having uid 1 we introduce a new permission. This is
placed in social_core so that we can use it in all modules building on
top of this.

Remaining tasks

User interface changes

API changes

Data model changes