Added a "Published in the last X days" filter, applied by the backend

GET /api/opportunities takes released_from, an RFC 3339 time, and lists
only the opportunities whose latest release is at or after it. It works
with paging and with changed_after, which keeps the same batch either
way; an invalid value is 400.

The frontend's filter defaults to 7 days, from midnight UK time, and
sends it with every load and catch-up, so a year of opportunities isn't
downloaded to show a week's. Changing the days loads the list again;
Clear filters goes back to 7.

internal/store/queries_test.go

@@ -124,7 +124,11 @@ var checks = map[string]queryCheck{
 	// opportunity_change_batch_opportunities
 	"insertChangeBatchOpportunity": {sql: insertChangeBatchOpportunity},
 	"selectOpportunitiesChangedBetween": {
-		sql: selectOpportunitiesChangedBetween,
+		sql: selectOpportunitiesChangedBetween + orderOpportunitiesChanged,
+	},
+	"orderOpportunitiesChanged": {
+		sql: selectOpportunitiesChangedBetween + "\nAND " +
+			filterOpportunityReleasedFrom + orderOpportunitiesChanged,
 	},
 }