Add character diff highlighting in emails on push service

• Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/10126
• Dev link: None
• GitLab version: 8.0

Email on push service does not have character-level diff highlighting. Customer requests that we add this capability so diff highlighting is in line with the web view.

Web diff view with character-level highlighting

Email on push current diff example

Next steps

Is this possible? What amount of work is necessary?

cc/ @DouweM