This essentially allows to pass both user and email, so that we can
either prefer the user to retrieve the avatar or (if user is not
present) fall back to the email lookup.

We want to drop the generic #avatar_icon helper that supports both an email
and a user object being passed in. Instead, we want to explicitly use the #avatar_icon_for_user
and #avatar_icon_for_email helpers depending on what we have at hand.
This allows us to avoid unnecessary database queries (e.g. call
User.find_by_any_email if we already have the user).

In situations like here, this makes it less convenient to use.

Whenever we already deal with a User object, let's use the more specific
method avatar_icon_for_user.

Show who triggered the pipeline. And also show the commit committer if
it's different from the author.

Builds in Ruby 2.1 specs were failing in master: https://gitlab.com/gitlab-org/gitlab-ce/builds/5258614

Closes #23498

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6019#note_16852207

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6019#note_16832930
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6019#note_16832936
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6019#note_16833033

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6019#note_16832366