[Rails5] Upgrade to 5.2

Currently in Gemfile we use Rails 5.0.7, but to avoid multiple bigger upgrades, we should investigate how much effort it takes make GitLab working ideally with 5.2, and if this is too much effort how difficult it would be to upgrade to 5.1.

We should also:

• fix lograge to play with Rails 5.2 properly: https://github.com/roidrage/lograge/issues/266 (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20630#note_137903623)
• consider updating activesupport in gitaly simultaneously (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8877#note_127482919)