Fix #493, make insert cell menu a split UI

After the change, the pop menu will only shows up when click on the down-pointing triangle on the right. If clicking on the left, text part `insert cell`, a code cell will be inserted. The evaluator for the new code cell is determined as commented in the code:
https://github.com/twosigma/beaker-notebook/commit/2add534995ebf2eaf02c3cbb839113ccef69ee69#diff-66bf8e92ebae0a71e23d29ea062a6675R94
Would this make sense to add to the notebook helper instead of putting this inside of the new cell directive?