Fix System Icons when closing a thread v2 (modmail-dev#65)

Co-authored-by: Taku <45324516+Taaku18@users.noreply.github.com>

Author: iamEvanYT

core/models.py

@@ -31,7 +31,7 @@ def __init__(self, app, data):
 
     @property
     def system_avatar_url(self):
-        return "https://discordapp.com/assets/" "f78426a064bc9dd24847519259bc42af.png"
+        return "/static/img/system-icon.png"
 
     @property
     def human_closed_at(self):