Transform incoming slack call URLs

See: https://github.com/dylex/slack-libpurple/issues/166

overlays/purple-slack/call-transformation.patch

@@ -0,0 +1,22 @@
+diff --git a/slack-message.c b/slack-message.c
+index de49fa6..f870a04 100644
+--- a/slack-message.c
++++ b/slack-message.c
+@@ -164,6 +164,17 @@ void slack_message_to_html(GString *html, SlackAccount *sa, gchar *s, PurpleMess
+ 		}
+ 		s = r+1;
+ 	}
++
++	if (strstr(html->str, "slack.com/call") != NULL) {
++		char find[200] = "";
++		strcat(find, sa->host);
++		strcat(find, "/call");
++
++		char replace[60] = "app.slack.com/free-willy/";
++		strcat(replace, sa->team.id);
++
++		g_string_replace(html, find, replace, 0);
++	}
+ }
+ 
+ /*

overlays/purple-slack/default.nix

@@ -11,6 +11,7 @@ self: super: {
 
     patches = (old.patches or []) ++ [
       ./browser-based-auth.patch
+      ./call-transformation.patch
     ];
   });
 }