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

main.go

@@ -28,6 +28,7 @@ func main() {
 
 	engine := html.New("./templates/"+config.Dashboard.Theme, ".html")
 	engine.AddFunc("divide", templateDivide)
+	engine.AddFunc("HTMLDateFormat", templateHTMLDateFormat)
 
 	app := fiber.New(fiber.Config{
 		Views: engine,
@@ -45,9 +46,7 @@ func main() {
 	})
 
 	app.Get("/accuracy-notice", func(c *fiber.Ctx) error {
-		return c.Render("accuracy-notice", fiber.Map{
-			"Defaults": config.getTemplateDefaults(),
-		}, "base")
+		return c.Render("accuracy-notice", config.templateDefaultsMap(), "base")
 	})
 
 	app.All("/admin", config.adminEndpoint)
@@ -58,9 +57,7 @@ func main() {
 				time.Sleep(time.Second)
 				os.Exit(1)
 			}()
-			return c.Render("restart", fiber.Map{
-				"Defaults": config.getTemplateDefaults(),
-			}, "base")
+			return c.Render("restart", config.templateDefaultsMap(), "base")
 		}
 		return c.Redirect("./", 307)
 	})