[TAV-353] Optimize table indexing.

There are a few coupled look-ups that we do so add those. Also, run_id in the
runs table is not a foreign key and we never search for it so we can remove that
one.
[TAV-353] Set inverse relationships in the models.
[TAV-353] Remove unused Paperclip columns.

In the runs table the type of the results file is *always* a zip and the type of
the log file is *always* plain text so we don't need these columns. We don't
care about when those things are updated so we don't need those columns.