users: Fix do_deactivate_user to save is_mirror_user.
users: Fix do_deactivate_user to save is_mirror_user.

This was a bug - is_mirror_user was not listed in update_fields despite
being changed.

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