Convert time stored in UTC to local timezone for picker

form.datetime_field uses html input `datetime-local` under the hood, which does not factor in
timezones, so convert it here explicitly

app/views/banners/_form.html.erb

@@ -28,7 +28,7 @@
     </div>
     <span class="input-style-1">
       <%= form.label :expires_at, "Expires at (optional)" %>
-      <%= form.datetime_field :expires_at, required: false %>
+      <%= form.datetime_field :expires_at, value: banner.expires_at&.in_time_zone(cookies[:browser_time_zone]), required: false %>
     </span>
     <div class="input-style-1">
       <%= form.label :content %>