Merge pull request #436 from twosigma/byu/fix-435

fix lock navigation y<0, prevent y<0 for bars&rivers, prevent stuck tran...
Fix #370, correct how to determine whether a seesion is savable
Merge pull request #438 from twosigma/alee/fix-370

Fix #370, correct how to determine whether a seesion is savable
Fix #254, strip off the shellIDs when saving notebooks
Merge pull request #439 from twosigma/alee/fix-254

Fix #254, strip off the shellIDs when saving notebooks
Move code-cell behaviors according to design

Implements a basic version of the codecell styling
that does not remove any functionality however brings it closer
inline with the implemented design.
Remove side click behavior from codecells
Move right cell menu according to new design.
Add a test for showing the codecell menu
Test toggling a code cells visibility
Move cell menu items into top level header
Update style of the new cell indicator.
Test expanding the newCell menu
Update dropdown menu styles.
Update new app header styles.
Add a compile task for scss
Replace app.css with app.scss
Add a watch task for dev
Split up styles into modules
Ignore npm-debug.log