Config overhaul: make some fields mandatory

[josecelano]

Some configuration options are mandatory. The tracker will panic if the user doesn't provide an explicit value for them from one of the configuration sources: TOML or ENV VARS.

The mandatory options are:

[metadata]
schema_version = "2.0.0"

[logging]
threshold = "info"

[core]
private = false
listed = false

[josecelano]

ACK 90ef14d2d5450421fd2b59b78e09536ed30152e7