Add new GitLab CI configuration facade class

## What does this MR do?

This MR is a first iteration of major CI configuration refactoring. See #17139 and #15060 for more details.

## What are the relevant issue numbers?

Closes #17139

See merge request !4462

Ability to prioritize labels

Closes #14189 

See merge request !4009

Workhorse to serve raw diffs

Fixes (partially) #13999

Dependent on: gitlab-org/gitlab-workhorse!45

See merge request !4130

Rename ci_commit in application code

This is third iteration of renaming `[ci_]commit(s)` in application code to `pipeline(s)`.


See merge request !4464

Rename Ci::Commit in specs

This is second iteration of renaming Ci::Commit occurences, this time I try to rename all `let's` and `factories` to match what type of object is created:

1. Rename factories: `ci_[...]_commit_[...]` to `ci_[...]_pipeline_[...]`,
2. Rename all `let's(:[ci_]commit)` to `let(:pipeline)`,
3. Rename all occurrences of `[ci_]commit` to `pipeline` if Pipeline object is created

cc @grzesiek @DouweM 

See merge request !4463