Migrate configuration handling to the confuse lib, take 2

[jinnatar]

This enables:

• Conservative config defaults in src/default_config.yaml, which leads to smaller and leaner custom configs.
• Config validation with confuse templates or at-use get methods with type checking.
• Easier future mutation of config format as defaults can carry over sane defaults.

This change touches all integrations and has minor potential for subtle breakage.

The following notifiers have been thoroughly tested:

• Telegram
• Pushover
• IFTTT
• MQTT
• SMTP
• Script
• Discord

Notifiers not tested:

• Slack

Notifiers with known issues:

• Grafana, WIP

Notifiers I cannot test myself:

• Pushcut

Groundwork for fixing #361

[martomi]

@Artanicus sorry, I might have merged while you’re still in WIP (just read the updated PR description). Feel free to finish the rest in a separate PR, as this one was already looking good and was quite large so I wanted to have it in master as a base for other changes :-)

[jinnatar]

No worries, all in all it's fairly solid. Slack I haven't tested yet (slightly more complex integration) but will, Grafana I know fails because it violates a few notifier conventions (have a WIP fix I'll PR separately since it's a bit opinionated), and Pushcut I can't test since they support such a limited set of platforms.