Repurpose `config.py` code as server class methods and controller functions

The functions in the old `config` module are mostly geared towards adding new TRS and WES server implementations to the application config — and fetching the relevant information as needed. If we're using a database, these functions can be replaced by simple write/read calls (`create`/`find` operations for Mongo). The methods for interacting with the db should probably be part of the `Server` class, whereas the `server_controllers` module can use those methods indirectly through some abstraction.