sync: Implement "Load More History" instead of "Load All"

[williamstein]

Currently with TimeTravel, you get the last few hundred edits (back to the last snapshot), and can click "Load All" to get the entire edit history of the document. However, if a document has been edited for a very, very long time, this "Load All" history might be too big to reasonably grab all at once. E.g., I have a todo list I've been using everyday a lot for years and:

Probably we should just replace "Load All" with loading the next chunk of history back to the previous snapshot. You can click it repeatedly to get more and more.

I'm going to label this a bug, since it is a situation that (1) causes heavy server load, and (2) makes it so you lose access to your data (=TimeTravel history) of your document.