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

templates/default/admin.html

@@ -17,6 +17,7 @@
     <h3>HomeAssistant</h3>
     <label>HomeAssistant's base URL <input type="text" name="base_url" value="{{.Config.HomeAssistant.BaseURL}}" required></label>
     <label>HomeAssistant's API Key <input type="text" name="api_key" value="{{.Config.HomeAssistant.ApiKey}}" required></label>
+    <lablel>Installation date<input type="date" name="installation_date" value="{{HTMLDateFormat .Config.HomeAssistant.InstallationDate}}" required></lablel>
 
     <h3>Sensors</h3>
     <label>Polled Smart Energy Summation entity ID <input type="text" name="polled_smart_energy_summation" value="{{.Config.Sensors.PolledSmartEnergySummation}}" required></label>