Add button to create a raw cell

[cindyytong]

Context

Today we support raw cells however you can only create a raw cell by:

1. making code/markdown and using the command r to convert or
2. using the cell navigation bar to convert to raw

How might we make this option more discoverable within the notebook navigation and in between cells

Ideas

1. In the notebook navigation bar make the +code button a dropdown with a raw cell type.
2. We may want the selected option to default to the last used

Inspiration