allow passing user's email address in custom Gravatar urls

Merged username-removed-15595 requested to merge arnaud/gitlab-ce:email_in_gravatar_urls into master

Some custom avatar systems use the user's email address rather than its hash. This merge request will allow the administrator to configure gitlab-ce to use the user's email address gravatar.plain_url and gravatar.ssl_url in the config file like this:

    enabled: true
    plain_url: "{email}&size=%{size}"
    ssl_url: "{email}&size=%{size}"

It's a only a 2 word patch.