Refactored database, mail and webhook notifications

[alexjustesen]

Warning

All other notification channels are deprecated and will be removed in a future version.

📃 Description

This PR refactors the core database, mail and webhook notification channels and establishes a pattern for Apprise in a future release. While there are some changes to the contents of the core notification channels this PR mostly cleans up the codebase.

• closes [Feature] Remove notifications on manual speedtests #2427

🪵 Changelog

➕ Added

• Added big deprecation notice to notification channels page.
• Notify the dispatching user via database channel when a speedtest completes, fails or when any benchmark fails.

✉️ Mail channel

• Unhealthy speedtest mail now shows all benchmarks with their status.

🌐 Webhook channel

• Unhealthy speedtest webhook now includes all benchmarks with their status.

✏️ Changed

• Moved core channels to tabs for easier navigation.
• Poll database notifications every 5s instead of 30s.
• When running a manual test only the dispatching user will be notified. All other notification channels are muted during a manual test.

🌐 Webhook channel

• Field metrics changed to benchmarks.

🔧 Fixed

• When a speedtest is unhealthy don't send a completed speedtest notification to reduce notification noise.

🗑️ Removed

🪣 Database channel

• When a benchmark fails only notify users once instead of per benchmark.