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
same as above, not sure if this logic belongs in this directive since it is operating over all the cells.

Why should an insert button reach into all the cells