Few fixes, improvements

Created by: artfulrobot

Hi Tim,

Thanks for publishing this extension. I'm about to start using it in the real world (!!) and have developed a few extras for it. Sorry to have munged a couple of issues together in on commit, would have been nicer to separate in case you don't like one of them...

1. There was - what looked like - a mis-named hook instance. Might have caused a name collision.

2. There was an issue when a Mosaico template was cloned/copied in that this was copying over the ref to the original message template, but it should create a new one.

3. The subject/title of the message template was set once at creation and could then not be edited at all (because of the is_reserverd flag). I've changed this behaviour so that these fields are updated any time the Mosaico mailing is edited, and also so that you can specify a separate msg_title and msg_subject in the Mosaico title text by separating the subject with a | symbol.

I checked your tests still worked and added tests for this new stuff. Take a look.

Hope you're well, Rich