Extend 'import data' routine to support event deletion

At the moment the only ways to bulk delete events is (1) manually, (2) via the API or (3) via a service desk request. The API method isn't particularly user friendly but could be nicely supplemented by implementing a process similar to the existing one for importing data. Presumably in the case of deletions the only required fields (to identify matching events) would be one or more of the usual user identifiers plus the start and end dates. Plus some suitably dire warnings about deletions being irreversible (but that is what 'mock runs' are for right?).