Add a maximum line length and image size in emails

## Issue Description

The text in email notifications sent out by H2 doesn't have a line length limit. This makes reading posts and replies on a laptop unpleasant and I often find myself having to click through to the post or comment to read it instead of reading it in my email client (Gmail web).

This also causes the size of inline images to blow out to fit the width, making then sometimes unnecessarily huge.

## Steps to reproduce:

On a laptop or larger screen, open an email notification which contains some long paragraphs. Observe the lack of line length limit that causes both long lines of text and large images.

## Expected Result:

There's a reasonable line length limit for text. This should in turn prevent the large images.

## Screenshot