Change configuration file strategy

[sseshan7]

Presently, a local file - config.py - is used to store keys and credentials needed for the web app to function. If others want to install this, they have to provide their own config.py file. This can be improved greatly.

I like @acxz's suggestion to store the creds in a json file, and have config.py read from that file, with sensible defaults, and descriptive error messages to show that creds have not been set.