Last_run in list_resources view is effectively the first run

When trying to come up with a fix for issue #326, I noticed some code in which all kinds of info about resources is gathered in a loop. In this code the first and last runs of all (filtered) resources is determined as well. However, I've noticed that the comparison operator is "less than" in both cased.

See the following code in views.py, function `list_resources`:
```py
for resource in response['resources']:
if resource.run_count > 0:
# View should work even without Runs
if first_run is None or resource.first_run < first_run:
first_run = resource.first_run
if last_run is None or resource.last_run < last_run:
last_run = resource.last_run
response['first_run'] = first_run
```

I've also noticed that, although first_run and last_run are stored in the response object, they don't appear to be used anywhere. This might also be the case for other variables collected in `list_resources` and other functions. Cleaning this up should also improve the performance, as reported in issue #326.

Beginne in views.py bei der Funktion list_resources und untersuche, wie first_run, last_run und die anderen gesammelten Werte in der Antwort verwendet werden. Korrigiere den Vergleich für den letzten Lauf und ermittle, welche nicht verwendeten Werte entfernt werden können; erledigt ist die Aufgabe, wenn die Ressourcenauflistung den korrekten letzten Lauf meldet, ohne ihre Antwort zu beeinträchtigen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.