Lock selenium-webdriver
Space out html output

fixes #586
Enforce min height on output cell

Fixes #546
Merge branch 'sjs/html-output'
Handle moving markdown editor up and down

fixes #350

Due to the way that epic editor works and the way that moving elements
via the cellmanger you need to reinit the markdown editor to reshow the
markdown content.
Fix whitespace growth bug

fixes #549
fixes #454