Rename CI::Commit to Ci::Pipeline

This is first iteration of renaming Ci::Commit to Ci::Pipeline, purely rename not a single change to tests:

1. Rename Ci::Commit to Ci::Pipeline,
2. Rename CommitStatus::commit to pipeline,
3. Rename Project::ci_commits to pipelines,
4. Rename Project::ci_commit to pipeline,
5. Rename Project::ensure_ci_commit to ensure_pipeline,
6. Rename MergeRequest::ci_commit to pipeline,
7. Rename Commit::ci_commits to pipelines,
8. Rename Commit::ci_commit to pipeline.

Next iteration will be renaming all ci_commit(s) to pipeline(s) in code and in tests.

cc @grzesiek @douwe