Fix #41: refactor download views

[rene-armida]

Some design notes:

• CSVDownloadView is a direct descendant of View and defines hooks for providing information about the CSV to be generated (e.g. its filename, column headings, etc.)
• One of these hooks, get_queryset, coordinates with the SearchableListMixin and SortableListMixin from extra_views
• because this method is shared with outside code, we cannot change its signature - it can only take a single argument, self.
• some of these CSV hooks need information from the current request, like URL parameters or path components. Thus, implementors should define their own get that extracts necessary info, and then forward on the call via super.
• for consistency, all of the other hooks also only take a single argument (instead, say, of being forwarded *args and **kwargs from the get method, which would be less annoying)

[rene-armida]

Also: the "balances" view is still not accessible from the nav bar, and still looks mostly broken, so although the spreadsheet is generated, I don't really know that it does the right thing (since it's hard to compare against the visual output of the normal, non-CSV page)