Newsletter tweaks

Could we get a few functionality tweaks for the newsletters:

- The ability to preview the newsletters before submitting them (so we can check if code is broken).
- The ability to edit them after they're submitted (in case there's an error we didn't catch earlier).
- The ability to disable "automatic lines" (so that tables and other code doesn't display weirdly).

I think implementing most or all of these features will help fix errors in newsletters.