Schema validation for config

[ethanwu10]

Is your feature request related to a problem? Please describe.
There is currently no validation of rCTF's configuration at server startup. This can result in misconfiguration going unnoticed until something else breaks later due to a bad config value.

Describe the solution you'd like
The configuration should be validated via a schema (probably with AJV since fastify uses it for route schemas). We may also want to look into specifying environment variables for config options via an extension to the schema, to reduce the boilerplate associated with parsing environment variables.