Toggle between markdown and code cell puts you into edit mode

[IanMatthewHuff]

Noticed this while validating the 'y' 'm' keyboard issue. In jupyter 'y' 'm' don't put you into edit mode, but in our tool we do. This is worse because if I try to toggle back I'm now entering code. We should not go into edit more on changing cell type.

[DonJayamanne]

Picking this as this is an area that will get modified as part of the ds/custom_editor work.
Would like to avoid dup work and merge conflicts as well.

[rchiodo]

This is still happening. 'y' and 'm' still put the user into edit mode.

[DonJayamanne]

Oh shoot, I was under the impression custom editor will be going in.
Since this touched some code that was getting modified in custom editor, I applied the fixes in custom editor branch.

[DonJayamanne]

I guess now we'd want to bring them into master... dang!

[rchiodo]

Validated