WIP settings system.

- updated dbdiagrams schema
- [BREAKING] force failure if `notify.filter_attributes` or `notify.level` is set
- added Settings table (and default values during migration)
- Added Save Settings and Get Settings functions.
- Added web API endpoints for getting and saving settings.
- Deprecated old Notify* constants. Created new MetricsStatus* and MetricsNotifyLevel constants.

webapp/backend/pkg/models/setting_entry.go

@@ -0,0 +1,22 @@
+package models
+
+import (
+	"gorm.io/gorm"
+)
+
+// SettingEntry matches a setting row in the database
+type SettingEntry struct {
+	//GORM attributes, see: http://gorm.io/docs/conventions.html
+	gorm.Model
+
+	SettingKeyName        string `json:"setting_key_name" gorm:"unique;not null"`
+	SettingKeyDescription string `json:"setting_key_description"`
+	SettingDataType       string `json:"setting_data_type"`
+
+	SettingValueNumeric int64  `json:"setting_value_numeric"`
+	SettingValueString  string `json:"setting_value_string"`
+}
+
+func (s SettingEntry) TableName() string {
+	return "settings"
+}