Fix problems with file based message templates and some admin JavaScript problems

Change the order in which message templates are loaded
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Message template overrides for specific newsletters were never used because "global" message templates were always loaded first.

Fix message preview template
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HTML preview of message was not shown due to condition depending on non-existing attribute message.newsletter.message_template.html.

HTML version of message is supposed to be optional. With database templates it depended on existence of HTML template but with current file based templates default HTML template for message always exist. So other mechanism is needed.

Fix JavaScript error on "Add submission" page
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On "Add submission" page following error occurred:
```
Uncaught exception: TypeError: Cannot convert 'submitlink' to object
Error thrown at line 6, column 8 in <anonymous function: init>(submitname) in http://127.0.0.1:8000/static/newsletter/admin/js/submit_interface.js:
    submitlink.href = 'javascript:SubmitInterface.process()';
called from line 177, column 64 in <anonymous function>(e) in http://127.0.0.1:8000/admin/newsletter/submission/add/:
    SubmitInterface.init('submitlink');
```

Template admin/newsletter/submission/change_form.htm always tried to add some JavaScript event for submitlink, even on "Add submission" page where there's no submitlink.

Fixed by wrapping relevant JavaScript in template with the same conditions that surround submitlink. But maybe JavaScript should be made more bulletproof?