Fix downtime check formatting

## What does this MR do?

This MR adjusts the formatting of the migration downtime checker so messages are more readable.

## Are there points in the code the reviewer needs to double check?

Not specifically

## Why was this MR needed?

Formatting was somewhat hard to read and the online/offline indicators used the wrong colour.

## New Format

```
[online]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20140407135544_fix_namespaces.rb
[online]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160705054938_add_protected_branches_push_access.rb
[online]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160705054952_add_protected_branches_merge_access.rb
[offline]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160705055254_move_from_developers_can_merge_to_protected_branches_merge_access.rb:

We're creating a `merge_access_level` for each `protected_branch`. If a user creates a `protected_branch` while this
is running, we might be left with a `protected_branch` _without_ an associated `merge_access_level`. The `protected_branches`
table must not change while this is running, so downtime is required.

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5081#note_13247410

[offline]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160705055308_move_from_developers_can_push_to_protected_branches_push_access.rb:

We're creating a `push_access_level` for each `protected_branch`. If a user creates a `protected_branch` while this
is running, we might be left with a `protected_branch` _without_ an associated `push_access_level`. The `protected_branches`
table must not change while this is running, so downtime is required.

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5081#note_13247410

[online]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160705055809_remove_developers_can_push_from_protected_branches.rb
[online]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160705055813_remove_developers_can_merge_from_protected_branches.rb
[online]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160716115711_add_queued_at_to_ci_builds.rb
[online]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160725083350_add_external_url_to_enviroments.rb
[online]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160727163552_create_user_agent_details.rb
[online]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160728081025_add_pipeline_events_to_web_hooks.rb
[online]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160728103734_add_pipeline_events_to_services.rb
[offline]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160729173930_remove_project_id_from_spam_logs.rb:

Removing a column that contains data that is not used anywhere.

[online]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160801163709_add_submitted_as_ham_to_spam_logs.rb
[online]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160802010328_remove_builds_enable_index_on_projects.rb
[online]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160805041956_add_deleted_at_to_namespaces.rb
[online]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160810102349_remove_ci_runner_trigram_indexes.rb
[online]: /home/yorickpeterse/Projects/gitlab/gitlab-ce/db/migrate/20160810142633_remove_redundant_indexes.rb
```

## Does this MR meet the acceptance criteria?

- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5840