Fixed the avatar problem when gitlab is deployed at the relative url.

The commit fixes the following problems:
The avatar is broken when the gitlab is deployed at the relative url.
(Gitlab-7.0-stable)
The avatars in the network diagrams are displayed incorrectly. (Before
Gitlab-7.0-stable)
What benefits of replacing  `gitlab_config.url` with `Gitlab.config.gitlab.url` ?