Remembering collapsed/expanded states of page and structure tree items

[dimaip]

https://twitter.com/Mindpilot1/status/920925064916324353

Do you think this is actually a good idea? Should we do it?

[mhsdesign]

STUB: The tree should remember which branches were opened/closed during the last time
This should be possible with dynatree's cookie functionality

from #3148