Relates to: torrust/torrust-tracker#401

We should apply the same improvements we applied to the Torrsut Tracker.

Subtasks (v3.0.0)

• Configuration overhaul: add tests for configuration #587
• Configuration overhaul: clean and reorganize before migration to Figment #589
• Configuration overhaul: migrate configuration to Figment #581
• Configuration overhaul: rename env vars #596
• Configuration overhaul: replace primitive types in configuration with richer types #599
• Configuration overhaul: define default values and remove default values from config file templates #603
• Configuration overhaul: remove support for env vars with old names #605
• Configuration overhaul: version 2 for the configuration toml file (breaking changes) #591