save "unsaved" graph in local storage

[kghmanuel]

Given the introduction of session time out, it is entirely possible that a graph can end up getting ignored and the progress is lost. having a copy in local storage may serve as a "temporary" storage of the most recent version of the graph. this behavior is similar to how notepad++ keeps unsaved files into some temporary file.

some behaviors worth noting:

• only applies to unsaved/un-exported version.
• auto-save frequency should be configurable or at least sooner than session timeout.
• should no longer apply if the credentials changed.