dashboard configuration knows about db_reader

Refactor dashboard database.yml to use seamless_database_pool and use dashboard_db_reader from the deployment config. Default development, staging, and test envs use the same db connection configuration for reader and writer, so are unaffected. 

Simply configuring the pool does not eg. send all reads to the reader. We are already ‘using’ the reader explicitly for some actions in ReportsController. There are various ways to configure the behavior of this (including various ways to deal with replication lag) which I’ll experiment with once we verify that the current baseline is working.

See also: https://github.com/bdurand/seamless_database_pool