email: Set List-Unsubscribe headers where we have unsubscribe links.
email: Set List-Unsubscribe headers where we have unsubscribe links.

Since we have a convention of using `unsubscribe_link` as the context
variable name, this will apply to all of our emails with confirmation
links.

hackerkid/zulip,punchagan/zulip,zulip/zulip,kou/zulip,zulip/zulip,andersk/zulip,zulip/zulip,zulip/zulip,hackerkid/zulip,andersk/zulip,kou/zulip,andersk/zulip,rht/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,punchagan/zulip,zulip/zulip,andersk/zulip,punchagan/zulip,eeshangarg/zulip,hackerkid/zulip,rht/zulip,kou/zulip,punchagan/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,hackerkid/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,punchagan/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip