It still causes too many false alarms.

Attempt to create pg_trgm extension in migration for test/dev environments

See merge request !3805

Ci::Commit becomes a Pipeline object

1. Ci::Commit receives context: ref, :tag.
1. One Ci::Commit describes a one Pipeline
1. Pipeline is created from `.gitlab-ci.yml`
1. Pipeline is a ordered group of builds
1. We test MR against Pipeline
1. Pipelines have a separate view (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3703)
1. Pipeline can be triggered from UI (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3703)
1. Later we change `Trigger -> TriggerRequest -> Build` to `Trigger -> Pipeline` (future)
1. We add a Pipeline Hook that will be triggered on Pipeline status change  (future)
1. We extend notifications to use `Pipeline Hook` to send summary on pipeline changes (future)

After merging that I'll prepare a separate MR that will unify naming, database columns, table names:
```
Ci::Commit -> Pipeline
Ci::Build -> Build
CommitStatus -> Job
GenericCommitStatus -> ExternalJob

ci_commits -> pipelines
ci_builds -> jobs
```

This MR implements first 5 points.

This is made to solve this issue https://gitlab.com/gitlab-org/gitlab-ce/issues/14149.

See merge request !3653

Closes #15210

Signed-off-by: Rémy Coutable <remy@rymai.me>

Add due_date text field to sidebar issue#show
Add ability sorting issues by due date ASC and DESC
Add ability to filtering issues by No Due Date, Any Due Date, Due to tomorrow, Due in this week options
Add handling issue due_date field for MergeRequest
Update CHANGELOG
Fix ambigous match for issues#show sidebar
Fix SCREAMING_SNAKE_CASE offenses for due date contants
Add specs for due date sorting and filtering on issues

update changelog

# Conflicts:
#	db/schema.rb