zephyr_mirror: Specify a z_charset of utf-8

andersk reports that we are "incorrectly sending outgoing zephyrs with z_charset
= ZCHARSET_ISO_8859_1, which breaks zwgc (other clients tend to ignore
z_charset). I'm assuming you're shelling out to zwrite; you need to use zwrite
-x UTF-8, or export LC_ALL=en_US.UTF-8."

(imported from commit 8abb28bf8acbe0b5aa99ca13faab24e1e554a031)

bots/zephyr_mirror_backend.py

@@ -598,7 +598,9 @@ Feedback button or at support@zulip.com."""
     wrapped_content = "\n".join("\n".join(wrapper.wrap(line))
             for line in message["content"].replace("@", "@@").split("\n"))
 
-    zwrite_args = ["zwrite", "-n", "-s", message["sender_full_name"], "-F", "Zephyr error: See http://zephyr.1ts.org/wiki/df"]
+    zwrite_args = ["zwrite", "-n", "-s", message["sender_full_name"],
+                   "-F", "Zephyr error: See http://zephyr.1ts.org/wiki/df",
+                   "-x", "UTF-8"]
 
     # Hack to make ctl's fake username setup work :)
     if message['type'] == "stream" and zulip_account_email == "ctl@mit.edu":