Remove instant preview from the snippet admin form.

The instant preview has a few issues:

- Occasional errors due to weird iframe back-forward behavior.
- Slowness in editing large snippets due to constant sending of snippet
  data, as well as from snippets that take up some CPU.
- Restrictions on functionality due to being in an iframe.

Thus I’ve removed the iframe and the instant preview and added a button
to trigger a preview in a new tab.