This reverts commit 3aaf4751e09262c53544a1987f59b1308af9b6c1, reversing
changes made to c6577e0d75f51b017f2f332838b97c3ca5b497c0.

- Is possible that gl_id could be a 'user_id'
- Is a more accurate variable name

This reverts commit a18c90128e4d3eeae1233b1bc3c3998afd223c0d, reversing
changes made to 0a64624152735766c428d1532e434dd0bf5a9748.

This commit adds the option of having another set of global custom hooks
along with the already supported repository local custom hooks.

The repository local custom hook is executed first (if available). If
successful, execution continues with the global custom hook (if available).
This way, local custom hooks get priority over global custom hooks.

Global custom hooks can be enabled by placing an executable file into the
"custom_hooks" directory within gitlab-shell (create if it does not exist,
yet).

This commit takes the GitlabCustomHook a bit clother to the other hook
handling classes by receiving the repo_path as argument to initialize()
instead of passing it to each method.

This reverts commit cd9e796316560eee596f2681d54a8417389bbe79.