Add auto-logout/save after long period of inactivity

[PropGit]

If leaving the BlocklyProp editor open for a while (with an existing program displayed, in this case), selecting to Compile or Load displays a NAN message.

I believe the real problem is that the user was automatically logged out between the last successful use and now, as navigating to the home screen, logging in, and opening the same program again works properly.

Messages like "NAN" do nothing to inform the user of the actual problem. We need BlocklyProp to trap this kind of error, determine the actual cause, and display a friendly message that leads the user to the solution. Such as "Can not compile because your account is logged out. Please log in and try again." in this case.

This issue may also be related to Issue #420.