Add bookmark menu to restore previous interact states
Bookmark UI fixes
Add _bookmark method to create bookmarks programmatically
Fixing error handling on interacts and linked cells

• Traceback output from an interact prints inside interact output block
• Kernel is not killed if non-default interact control values or
  linked cells trigger an exception
Print interact state when an exception occurs (fixes #425)
Allow the user to create permalinks with interact values and bookmarks

(fixes gh-404)
Merge remote-tracking branch ‘upstream/master’ into bookmark

Conflicts:
	interact_sagecell.py
Fix indentation error
Make the permalink generator retry in the event of a collision
Only store non-nested interacts in permalinks
Merge remote-tracking branch ‘upstream/master’ into bookmark

Conflicts:
	handlers.py
	static/compute_server.js
Update Sage patches for Sage 5.11
Add a button to export interact bookmarks as text
UI tweaks
Merge pull request #421 from ihanson/bookmark

interact save state/load state