Backend: Rename CI classes to better reflect CI domain

Proposal

• Rename Commitstatus to Ci::Job
• Rename GenericCommitStatus to Ci::ExternalJob

Old proposal

We want to introduce multi-project pipelines soon, but this requires some minor architecture changes of underlying domain model, to make it easier to implement a new feature.

Changing few things would simplify the implementation:

• extract common CI job features from CommitStatus to Ci::Job abstract class
• rename GenericCommitStatus to Ci::ExternalJob < Ci::Job
• extract general abstract implementation from Ci::Build to Ci::Job
• inherit from newly created abstract class in Ci::Build < Ci::Job

This will make it easier to add new job types like Ci::PipelineBridge or Ci::PipelineDependency that would make it easier to implement multi-project pipeline.

Alternative naming convention:

• Ci::Job module
• Ci::Job::Base abstract class
• Ci::Job::External external job
• Ci::Job::Build build
• Ci::Job::Bridge or Ci::Job::Dependency for pipeline trigger/depedency (or both for different dependency direction)

/cc @ayufan