Add proper labels to shift date selectors

We were using one label for both date selectors, but it wasn't
associated with any of them.

So we're now rendering one label per control and, just like we only show
one of these date selectors at a time, we're only showing one label at a
time.

app/components/admin/poll/shifts/form_component.html.erb

@@ -20,11 +20,18 @@
     </div>
 
     <div class="small-12 medium-3 column">
-      <label><%= t("admin.poll_shifts.new.date") %></label>
+      <%= label_tag :shift_date_vote_collection_date,
+                    t("admin.poll_shifts.new.date"),
+                    class: "js-shift-vote-collection-dates" %>
       <%= select "shift[date]", "vote_collection_date",
                  options_for_select(shift_vote_collection_dates),
                  { prompt: voting_polls.present? ? t("admin.poll_shifts.new.select_date") : t("admin.poll_shifts.new.no_voting_days") },
                  class: "js-shift-vote-collection-dates" %>
+
+      <%= label_tag :shift_date_recount_scrutiny_date,
+                    t("admin.poll_shifts.new.date"),
+                    class: "js-shift-recount-scrutiny-dates",
+                    hidden: "hidden" %>
       <%= select "shift[date]", "recount_scrutiny_date",
                  options_for_select(shift_recount_scrutiny_dates),
                  { prompt: t("admin.poll_shifts.new.select_date") },