Refactor email templates

theme/templates/email/trades/trade_update_sent.txt

@@ -0,0 +1,15 @@
+{% include 'email/common/header.txt' %}
+
+{{ acting_user }} ({{ acting_user_ign }} {{ acting_user_friend_code }}) has marked your trade as "Sent".
+
+Trade Details:
+- You have: {{ want_card }}
+- You want: {{ has_card }}
+(#{{ hash }})
+
+What's next? Once you respond to the trade in the app, please mark the trade as "Received" in your dashboard.
+
+Visit your dashboard to manage this trade:
+{{ domain }}{% url 'dashboard' %}
+
+{% include 'email/common/footer.txt' %}