Support for Gmail's "conversation thread" formatting

[Ted Goas]

As reported in Stacks Email, Gmail web client will attempt to highlight portions of the email body that it believes to be important parts of a previous thread. It does this by applying an .im class to the portion of the body and highlights text in purple.

While it's good to know why it's happening, we can't prevent it from never happening. So let's include some defensive code in the templates to intercept it and make Gmail's perceived conversation text appear visually consistent with the rest of the text.