Adding security.js with 1st attempt at is_safe

The goal of this PR is to detect if Markdown is safe from a security standpoint and refuse to render it if it is.

Tasks:

- [x] Implement `IPython.security.is_safe`.
- [ ] Add logic to `TextCell` and subclasses to use `is_safe`.
- [ ] If `TextCell` content is unsafe, display a warning instead of the content.
- [ ] Add tests for all this.