When I look at a page after a long time, check whether it is obsolete

Summary: Currently, it is annoyingly easy to go back to an old page, do a bunch of data entry, save or move on -- and lose my work because the page version is too old, so it reloads instead. We need to make this better.

While we probably can't fix this completely until we move over to a WebSocket architecture, we could put in a useful heuristic: whenever we front the page, check the timestamp of when it was loaded or last checked. If it is longer than some epsilon (certainly no more than an hour, possibly as short as a minute), ping the server with a null query that will cause a reload if the system version is too old.

In theory, we could also do something similar with the Space version of the page, which is intriguing, but might get annoying (since it will cause much more frequent reloads), so think about that one more carefully.