Dumb-down avatar presence check in `avatar_url` methods

`avatar.present?` goes through CarrierWave, and checks that the file
exists on disk and checks its filesize. Because we're hitting the disk,
this adds extra overhead to something where the worst-case scenario is
rendering a broken image.

Instead, we now just check that the _database attribute_ is present,
which is good enough for our purposes.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/19273