Enables scroll to bottom once user has scrolled back to bottom in job log

Closes #34846 and #39109

See merge request gitlab-org/gitlab-ce!15260

(cherry picked from commit a030a4a8)

ce86fba0 Enables scroll to bottom once user has scrolled back to bottom in job log
62527afd Merge branch 'master' into 39109-reenable-scroll-job
0b5eda18 Improve naming and cache variables