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