msg actions: Use real `Message` type on message objects.
msg actions: Use real `Message` type on message objects.

This makes the various grab-bag types we have here more similar to
each other, in preparation for consolidating them with each other.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile