Instead of using funtion `error!` the function `not_found!` is used to return 404 error. Adjusted documentation accordingly.

Accessing a repository branch that does not exist returns a 404 error instead
of 200 now. Added a test.

fix the gitlab-shell clone url in master installation doc

Add groups api

Update step 5 "database" (Issue #2894)

Made it a bit clearer what step 5 is for to stop people skipping it. This time with a relative link.

Don't setuid the repositories on installation