Move towards selectable time ranges in the dashboard

- Add the installation date to frontend and backend
- Add an error page to help with the upgrade from the previous version
- Avoid querying history if installation date is not set
- Make the fillMissing function work for periods of different lenght than than 8 days

config.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	"regexp"
 	"strings"
+	"time"
 )
 
 type Config struct {
@@ -19,8 +20,9 @@ type Config struct {
 }
 
 type HomeAssistant struct {
-	BaseURL string `json:"base_url"`
-	ApiKey  string `json:"api_key"`
+	BaseURL          string    `json:"base_url"`
+	ApiKey           string    `json:"api_key"`
+	InstallationDate time.Time `json:"installation_date"`
 }
 type Sensors struct {
 	PolledSmartEnergySummation string `json:"polled_smart_energy_summation"`