Monitor: Add an illustration to the top of the email to make it similar to others

Currently the emails we send out for Monitor notifications are a wall of text:

All of our other emails include an illustration to help convey the information in the email, and it seems like it'd be nice to have one here as well.

For the _site up_ email specifically, we might consider displaying an mshots screenshot of what the site looks like once we detect it back online? Although I think we'd have to store it somewhere, and that might introduce new problems around the screenshot vs what people expect.

Start by reviewing the Monitor notification emails, especially the site up email, and compare them with the other emails that already include illustrations. Define an illustration treatment for the top of the email, resolve whether an mshots screenshot is appropriate and how it would be stored, and consider the work done when the Monitor emails no longer appear as a wall of text.

Written by the indexing model from the issue text.