Previously, the email mirror queue worker used the API bindings to send
messages to Zulip, as if it were any other API client.
This is inefficient since we're running the worker inside the Django
context on a machine with database access; we can instead just use the
internal message-sending functions we use elsewhere. This also resolves
potential issues with SSL certificates, etc. that might occur when we
were previously making a HTTPS connection.
(imported from commit 6de8015829bec440f1af0199a2138828e86ed2a4)