Add syntax colors to diffs in email-on-push notifications

## What does this MR do?
This MR adds syntaxhighlighting to the email-on-push notifications.
## Are there points in the code the reviewer needs to double check?
No
## Why was this MR needed?
Currently the diff in email-on-push notifications is just a monochrome diff that's hard to read.
This MR adds colored syntaxhighlighting to the mails.
## What are the relevant issue numbers / Feature requests?
* It closes https://github.com/gitlabhq/gitlabhq/issues/6994
* There is a Discussion in the Feedback Forum: http://feedback.gitlab.com/forums/176466-general/suggestions/5802256-add-color-to-diffs-in-email-on-push-service-to-htm
* A pull request for this already existed, but was closed. https://github.com/gitlabhq/gitlabhq/pull/7739

## Screenshots
see prior Pull Request for a screenshot

See merge request !295