The classification filter now offers only the loaded opportunities' classifications

It lists the classifications the opportunities in the days window have,
not every one stored, plus any already chosen, so a selection stays
visible and can be removed when nothing in the window has it. The other
filters don't narrow it, so choosing a status doesn't make
classifications vanish.

web/README.md

@@ -69,8 +69,9 @@ again.
 
 The list is fetched a page (100) at a time until there are no more. The marks
 are `interested`, `skipped`, `bid_submitted`, `lost`, `won` and `hidden`;
-Won and Lost exclude each other (409). The classifications are the ones the
-filter offers: only those some opportunity has. The client fetches the CSRF
+Won and Lost exclude each other (409). The classifications are every one some
+stored opportunity has; the filter offers only those the loaded opportunities
+(the days window) have, and any already chosen. The client fetches the CSRF
 token when it first needs it, and a new one if a request is refused for it.
 
 ## Where things are