Validate configuration on startup

[briehl]

Things to verify on app startup that aren't now:

1. Service auth token (new variable in CE-394 Add service token #224) is valid.
2. Configured data paths exist and are either directories or files.

The right place to do this would probably be in the main startup entrypoint (i.e. main.py) then pass both the config and validated auth client to the webapp. Data path checking should be done in the config initializer.

While we're at it, this logic should probably live in a separate initialization function, and not just in a bare script in main.py, to allow for more complete testing.