Infrastructure for creating and using dumps of NCS codes.

This is a higher-performance replacement for loading the codes from
YAML when starting up a test suite.
Use CodeListLoader#load_from_pg_dump where appropriate.

Note that "where appropriate" does not include actual version
migrations. For those, we need to use `load_from_yaml` because it does
not truncate the table as the first step.

For running a single low-complexity focused spec, this change shows an
80% reduction in the time spent in RSpec — it decreases the time from
2.5s to about 0.5s. The wall-clock reduction is less impressive (20s
-> 18s) because it doesn't address the high overhead of loading Cases'
bundle.
Merge pull request #95 from NUBIC/code_list_load_from_dump

Load ncs_codes for tests using pg_restore instead of a bunch of YAML-derived INSERTs