search: fix server-side date behaviour

OffsetMinutesFromUTC was being passed incorrectly by the web app
(getTimezoneOffset if UTC - Timezone, we wanted Timezone - UTC), now
fixed. This value is now used if given in comparisons. Times are
truncated to minute-deep precision, and Any date comparison ignores
empty date fields (i.e. a unix timestamp being 0 or a time.Time.IsZero()
== true).

ts/modules/search.ts

@@ -219,7 +219,7 @@ export class DateQuery extends Query {
         }
         let out = parseDateString(valueString);
         let isValid = true;
-        if ("invalid" in (out.date as any)) { isValid = false; };
+        if (out.invalid) isValid = false;
         
         return [out, op, isValid];
     }