Determine whether user is reading using html or plain text

[mlandauer]

We can tell this in two ways:

• If an email is open tracked
• If an html link is clicked

And we can tell at the same time if image loading is turned off in html because that would show up as an html link being clicked while no open tracking taking place.

Far from perfect but hopefully can give some insight.

[schlos]

Linked to #74