PR: Allow Admins to edit system message content (fixes #549)

Hello @davefp

I made up some new features.
Preview for message is working but I don't know if is is a good way. For texts editor I use CKEditor, I have some problems with showing this item (for now to see this editor you have to reload page. But it is CKEditor but, and I was thinking how can I fix it. I do not pick up another text editor because in other editors it is not possible to color text).
For creating template I use Mustache, and thank you very much for that choice. All templates for are stored in Form model. Exchanges messages were done to coupe with many languages. This feature is also done: in Bids messages, and in Exchanges.
Tomorrow I look up how to fix problem with disappearing CKEditor, and add tests to this features.

Once again, thanks for all things, and I am waiting for comments, and notes.