Modified times aren't kept up to date in a draftset's modified times graph as live data changes.

[ricroberts]

With the changes from #506 around tracking modified dates, modified times aren't kept up to date in a draftset's modified times graph as live data changes.

This means that someone querying the modified times graph to to know when graphs in the union-live graph set have changed will need to query both live and draftsets's modified times.

This could do with being improved so that times are kept up to date in a draftset's modified times graph, to make it more natural when using union-with-live.

[lkitching]

Re-opening because I don't think that has been done(?). Changes merged into live are not merged back into other draftsets, so the modified time in live could be more recent than a draft change.