Avoid having duplicate events

[mst]

We probably run the web site data 'scraping' more than once for one website to gather updates.
We need a mechanism to determine, if one event is already in the database.

I would expect the name of events to stay the same, so name and date should be enough for now.
However, if we start scraping different sites, the other site might use different names for the same event. Same for the location.