Extract the first 3 chars during 3pid obfuscation, not just the 3rd char (#317)

This is a duplicate of #316 for mainline which fixed the same bug on `dinsic`. The bug originally stemmed from #311.

changelog.d/317.bugfix

@@ -0,0 +1 @@
+Fix a regression in v2.2.0 where the wrong characters would be obfuscated in a 3pid invite.

sydent/http/servlets/store_invite_servlet.py

@@ -169,7 +169,7 @@ class StoreInviteServlet(Resource):
         # If the string is shorter than the defined threshold, redact based on length
         if len(s) <= characters_to_reveal:
             if len(s) > 5:
-                return s[3] + u"..."
+                return s[:3] + u"..."
             if len(s) > 1:
                 return s[0] + u"..."
             return u"..."