Define AccountDirectory as an abstract backend for user data

Eventually this will fully support all account information, similar
to GroupBackend. For now lets start by defining a simple base class
that can supply name, email and avatar URLs to an AccountInfo for
the REST API. Bind this in the Daemon and web application code so
its at least partially pluggable at the static Guice injector level.

Change-Id: I6e1b55fd46f3e99a982f7bd523f3e9d91b637b0d
5 files changed