* Shuffle code around, adding UnsentTextAttachment
* Replace usages of TextAttachment with UnsentTextAttachment where applicable, creating an actual TextAttachment (and associated OWSlinkPreview image TSAttachment, if applicable) only per-recipient
* Only upload one text story link preview image attachment and propagate it to the individual TSAttachments created for each destination
* update comments so I don't lose my mind the next time I read this code
* Dedupe text stories sent to the same person via multiple private threads
* fix non-compiling tests
* pr feedback
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