Avoid Status.DoesNotExist during loaddata; document PostgreSQL to MySQL data migration

[glennmatthews]

Fixes: #555

During nautobot-server loaddata, the attempt in Cable.__init__ to cache any pre-existing Status for the cable was throwing an exception because the Status models hadn't been written to the database yet. Since Cable._orig_status is only used in the update_connected_endpoints signal handler for post_save, and only in the case where created == False, we can safely skip setting _orig_status in the case where the Cable is being newly created and doesn't pre-exist in the database.

That's my working theory anyway - this change definitely allows invoke loaddata to run without errors now, but our unit test coverage for this signal path appears pretty close to zero, so please hold me accountable here. :-)

[glennmatthews]

Backported #953 to develop so that we can easily use MySQL there for testing. Added notes about setting the MySQL character set and collation explicitly to ensure case-sensitivity and compatibility with Postgres. Add documentation about migrating data from PostgreSQL to MySQL - this could probably be fleshed out more thoroughly but the commands given work in my testing.