Bug 40664: Serial subscription input missing "Required" labels

This patch adds missing "Required" labels next to two mandatory fields
in the second page of the new serial subscription form.

To test:
1. Go to Serials
2. Click 'New subscription'
3. Enter a biblionumber in the 'Record' field (e.g. 230)
4. Click 'Next'
5. (Click 'OK' to confirm that you are not adding a vendor)
--> On the page 'Add a new subscription (2/2)' the fields 'Frequency' and 'Subscription start date' are red to indicate that they are mandatory, but there is no 'Required' note next to them.
6. Apply patch and refresh the page
--> It should say 'Required' next to all the mandatory fields (in red)

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Lisette Scheer <lisette@bywatersolutions.com>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit 4bb7a1371c)
Signed-off-by: Paul Derscheid <paul.derscheid@lmscloud.de>
(cherry picked from commit f90a00164b)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>

koha-tmpl/intranet-tmpl/prog/en/modules/serials/subscription-add.tt

@@ -324,6 +324,7 @@ fieldset.rows table { clear: none; margin: 0; }
                                                 </option>
                                             [% END %]
                                         </select>
+                                        <span class="required">Required</span>
                                     </li>
                                     <li>
                                         <label for="subtype">Subscription length:</label>
@@ -362,6 +363,7 @@ fieldset.rows table { clear: none; margin: 0; }
                                     <li>
                                         <label for="from" class="required"> Subscription start date:</label>
                                         <input type="text" size="10" id="from" name="startdate" value="[% startdate | html %]" class="flatpickr required" data-date_to="to" required="required" />
+                                        <span class="required">Required</span>
                                     </li>
                                     <li>
                                         <label for="to">Subscription end date:</label>