Re-added assetable on attachments

[johannlejeune]

Basically a revert of 4992843 since I didn't quite understand why it was removed in the first place and I think it can still be useful to a lot of people.

[dbourguignon]

I don't understand the remove of assessable either.
It's a breaking change

I will try your fork for now

@galetahub any backstory for this? I'd like to understand

[botularius]

I also don't understand the remove of assetable. As I did not see this pull request before I opended an issue here:
#788

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[januszm]

Per @galetahub comment: #683 (comment)

It looks like idea was to adapt CKeditor to the change in Rails where belongs_to is required by default, but the chosen solution is not ideal becase the feature, instead of being actually adapted to be 'optional' was simply removed.

[januszm]

But just to be fair, perhaps this feature was used so rarely and can potentially be implemented outside the scope of this gem that it was better to remove it. In all my projects, I realized, I haven't used it so it's not really a breaking change for me.