feat(postcard): add optional send_time field to received card form

app/templates/postcard_form.html

@@ -23,6 +23,9 @@
     <label>{{ 'pc_form.sender'|t(user.language) }} *</label>
     <input type="text" name="sender_name" value="{{ postcard.sender_name if is_edit else '' }}">
 
+    <label>{{ 'pc_form.send_time'|t(user.language) }}</label>
+    <input type="date" name="send_time" value="{{ postcard.send_time.strftime('%Y-%m-%d') if is_edit and postcard.send_time else '' }}">
+
     <label>{{ 'pc_form.receive_time'|t(user.language) }} *</label>
     <input type="date" name="receive_time" value="{{ postcard.receive_time.strftime('%Y-%m-%d') if is_edit and postcard.receive_time else today }}">