install: Remove references to "Zulip Voyager".

"Zulip Voyager" was a name invented during the Hack Week to open
source Zulip for what a single-system Zulip server might be called, as
a Star Trek pun on the code it was based on, "Zulip Enterprise".

At the time, we just needed a name quickly, but it was never a good
name, just a placeholder.  This removes that placeholder name from
much of the codebase.  A bit more work will be required to transition
the `zulip::voyager` Puppet class, as that has some migration work
involved.

scripts/setup/initialize-database

@@ -32,7 +32,7 @@ cd "$THIS_DIR/../.."
 ./manage.py migrate --noinput
 ./manage.py createcachetable third_party_api_results
 
-if ! ./manage.py initialize_voyager_db; then
+if ! ./manage.py initialize_production_database; then
     set +x
     echo
     echo -e '\033[32mPopulating default database failed.'