#32 overhaul db host/instance/user config

[Reece Hart]

The current configuration mechanism is fragile, confusing, and inflexible. It should be overhauled to achieve the following:

- clarity
- automatic configuration for within Genentech and the public db, and extensible to other sites
- support for aux configuration files
- pull config out of Page.pm and put (some) into DBI.pm