Ensure that vault content signing reference previous signatures

[willjasen]

Make sure that the signatures made that are placed into ".tephramesh/signatures" have data in each that link it to the file's previous global signature.

In the event that two devices make a change to a note at basically the same time, they would sign their changes of the same note, publish their signature which would link back to the same "contentHash", and the changes could potentially be merged if there's a conflict.