Use lowerCamelCase for the JSON API (fixes #1338)

Replace the current mix of UpperCamelCase und lowerCamelCase with
consistent lowerCamelCase keys for the JSON API. Also adapt the frontend
so it works with the changed API.

Attention: this will break existing consumers of the API.

internal/stats/device.go

@@ -24,7 +24,7 @@ import (
 )
 
 type DeviceStatistics struct {
-	LastSeen time.Time
+	LastSeen time.Time `json:"lastSeen"`
 }
 
 type DeviceStatisticsReference struct {