Add a RuboCop cop / spec for sanitize_name helper in emails

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Close this issue

In https://gitlab.com/gitlab-org/gitlab-ce/issues/22076 we added a sanitize_name helper for emails that should be used everywhere in an email that the user's name appears without being part of a link.

The problem is that it's very hard to audit this, so we should add some kind of automatic check to cover ourselves in future. (That is out of scope of the original security issue.)