Add thread, category and is_orphaned fields to attachments

[rafalp]

thread and category fields will let us easily move attachments around.

Making thread, user, category and post fields nullable will let us delete attachments at later time so we don't slow down delete operations with IO to delete files from media storage.

is_orphaned field will let us flag which attachments to delete.

This is a blocker for #1811 and new threads moderation actions.

[rafalp]

I've renamed is_orphaned to is_deleted in final implementation.