* Add newline to user organization spec according to test guide
* Remove unnecessary comments from user organization database migration

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

- Don't use `TableReferences` - using `.arel_table` is shorter!
- Move some database-related code to `Gitlab::Database`
- Remove the `MergeRequest#issues_closed` and
  `Issue#closed_by_merge_requests`  associations. They were either
  shadowing or were too similar to existing methods. They are not being
  used anywhere, so it's better to remove them to reduce confusion.
- Use Rails 3-style validations
- Index for `MergeRequest::Metrics#first_deployed_to_production_at`
- Only include `CycleAnalyticsHelpers::TestGeneration` for specs that
  need it.
- Other minor refactorings.

Incorrect syntax.

1. Change multiple updates to a single `update_all`

2. Use cascading deletes

3. Extract an average function for the database median.

4. Move database median to `lib/gitlab/database`

5. Use `delete_all` instead of `destroy_all`

6. Minor refactoring

1. Add indexes to `CreateMergeRequestsClosingIssues` columns.
2. Remove an extraneous `check_if_open` check that is redundant now.

It would've been better to rebase this in, but that's not possible
because more people are working on this branch.