Fix MR 500 when viewing the MR page

I did spot a two of problems:

1. ci_build_details_page had different implementation in EE which lead to the 500
2. I did deprecate the GitLabCiService, but it could still be picked as "active" CI Service due to fact that had active flag set

The both problems are resolved. The mentioned part of code is cherry-picked to GitLab CE codebase.

@rspeicher You will get a merge conflict when you try to merge it back to GitLab EE, but the fix is simple.

/cc @dblessing

Fixed #2712 (closed)

Title changed from Fix MR 500 when viewing the builds page to Fix MR 500 when viewing the MR page

mentioned in issue #4127 (closed)

Added 1 commit:

• 09008026 - Move ci_build_details_path helper spec to correct location

@ayufan The specs were in the wrong file and also wouldn't have passed as they were written.

mentioned in commit 25bf3eb8

Status changed to merged

Picked into 8-3-stable.

mentioned in commit c4eacbd2