settings: Move the retention days input below the dropdown.

Moved the "Retention period (days):" under the dropdown "Custom".

This is consistent with similar date window inputs elsewhere, and
conveniently just requires changing a single class.

Fixes: #23532

static/templates/settings/organization_settings_admin.hbs

@@ -81,7 +81,7 @@
                         <option value="custom_period">{{t 'Custom' }}</option>
                     </select>
 
-                    <div class="dependent-inline-block">
+                    <div class="dependent-settings-block">
                         <label for="id_realm_message_retention_custom_input" class="inline-block realm-time-limit-label">
                             {{t 'Retention period (days)' }}:
                         </label>