Add blind options to attach PDFs and webpage snaphots to the current item

[dstillman]

https://forums.zotero.org/discussion/comment/504284/#Comment_504284

And show an error if a valid item isn't selected

This would be a stopgap option until we had either SSE or native messaging and can show these options only when they were actually relevant (and show the title of the item).

[adomasven]

Not great on Firefox given that we have no progress window there, since we cannot show any descriptive error, so maybe needs #513 too.