signups: Use internal_send_stream_message().

We prefer this to internal_send_message().

We are trying to deprecate `internal_send_message`,
which has extra moving parts related to
`extract_recipients` and `Addressee.legacy_build`.

There are two chunks of code that I touch here
that look pretty similar, but I'm not quite
sure they're worth de-duplicating, since they
use different topics and different message
content.

tools/linter_lib/custom_check.py

@@ -390,7 +390,7 @@ python_rules = RuleList(
              # This one in check_message is kinda terrible, since it's
              # how most instances are written, but better to exclude something than nothing
              ('zerver/lib/actions.py', 'stream = get_stream(stream_name, realm)'),
-             ('zerver/lib/actions.py', 'get_stream("signups", admin_realm)'),
+             ('zerver/lib/actions.py', 'return get_stream("signups", realm)'),
          ]),
          'description': 'Please use access_stream_by_*() to fetch Stream objects',
          },