Fixed missing attachments from primary in secondary

[tipue-dev]

The article attachments are not carried over to the secondary if an internal note is created in the primary.
This also means, that the article itself in the secondary is plain text instead of HTML.

[rkaldung]

I labeled this PR as a feature because the behavior is new to the add-on. I checked it with a 6.0.x

Question: Should a system configuration setting added to keep the "original" behavior? I think some systems not to want to have attachments replicated to all secondary tickets.

[tipue-dev]

@rkaldung I think that in every other case, attachments are copied over. A system configuration that only excludes attachments for internal notes looks strange, if seen on its own.

For existing users, I don't imagine that agents expect attachments for other articles to be transferred but not internal notes (from prior experience, yes, but not because this is expected behaviour). I also assume those articles will probably have less attachments in general, so I don't see an argument for memory usage here also.

If you think memory usage might be a concern for some users, then I would propose a general solution that allows it to load attachments from other tickets. All Secondary articles could load all attachments from the associated Primary article. Ticket split could use the same.