Adds a relationship Message->PosterEmail

The Message#poster_email attribute gets set if the from_ address
corresponds to a PosterEmail.
Adds the joinedload() options

This optimizes the number of queries, as I initially fetch all the
tables I know I am going to need.

Note that the query for threads in the view 'vote' turns crazy, this is
because of the LIMIT clause that targets the number of threads and not
the number of rows in the rendered query.
Fixes querying posters from a thread

We already have this info deep within our list of threads, no need to
query again.