This is a first step to decouple pipeline from YAML processing. It
reduces the coupling by removing some methods that introduce coupling
and by moving logic into separate chain element that is being used to
populate pipelines with stages and builds.

This appears to reduce the number of queries in pipeline creation of
gitlab-ce's .gitlab-ci.yml by 92

This also means we now need to mock this ci yaml file properly in the
test because otherwise the yaml_errors stay on the object and make all
the pipelines failed.

- Move the exception handling as close to the source as possible to
avoid leaking Psych ahstraction
- Also remove unnecessary rescue all statement from LintsController.
This should not be necessary anymore since any YAML errors should all be
caught by the #validation_message method.

This should reduce the number of SQL queries and lookups needed to look up a
project of a build and pipeline and vice versa.

Before:

```
[1] pry(main)> Ci::Build.reflect_on_association(:project).has_inverse?
=> false
[2] pry(main)> Project.reflect_on_association(:builds).has_inverse?
=> false
[3] pry(main)> Ci::Pipeline.reflect_on_association(:project).has_inverse?
=> false
[4] pry(main)> Project.reflect_on_association(:pipelines).has_inverse?
=> :project
```

After:

```
[1] pry(main)> Ci::Build.reflect_on_association(:project).has_inverse?
=> :builds
[2] pry(main)> Project.reflect_on_association(:builds).has_inverse?
=> :project
[3] pry(main)> Ci::Pipeline.reflect_on_association(:project).has_inverse?
=> :pipelines
[4] pry(main)> Project.reflect_on_association(:pipelines).has_inverse?
=> :project
```

Uses `list_commits_by_oid` on the CommitService, to request the needed
commits for pipelines. These commits are needed to display the user that
created the commit and the commit title.

This includes fixes for tests failing that depended on the commit
being `nil`. However, now these are batch loaded, this doesn't happen
anymore and the commits are an instance of BatchLoader.

This changes Ci::Pipeline#latest_builds_with_artifacts so it returns an
Array instead of a relation. Whenever we use this data we do so in two
steps:

1. Count the number of rows
2. If this number is greater than 0, iterate over the rows

By returning an Array instead we only execute 1 query of which the total
time/work is less than running either just a COUNT(*) or both queries
(in the worst case).

On GitLab.com this change should save us a few milliseconds per request
to ProjectsController#show.

Adds a rubocop rule (with autocorrect) to ensure line break after guard clauses.

This adds an optimised way of getting the latest pipeline status for a
list of Commit objects (or just a single one).

Without this fix, pipeline could go from skipped to
running directly, bypassing the transition for:

    [:created, :pending] => :running

And this is responsible for setting up started_at.
Without this fix, started_at would never be set.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/39884

Use Commit#notes and Note.for_commit_id when possible to make sure we use all the indexes available to us