Cleanup Ci::Commit, Ci::Build and CommitStatus views

This MR tries to do first sweep of cleanups to Ci::Commit and Ci::Build objects removing all view-related functions and fixing the API from other side.

Fixes: https://gitlab.com/gitlab-org/gitlab-ce/issues/6046

/cc @grzesiek @DouweM

spec/models/ci/commit_spec.rb

@@ -23,7 +23,7 @@
   let(:commit) { FactoryGirl.create :ci_commit, project: project }
 
   it { is_expected.to belong_to(:project) }
-  it { is_expected.to have_many(:commit_statuses) }
+  it { is_expected.to have_many(:statuses) }
   it { is_expected.to have_many(:trigger_requests) }
   it { is_expected.to have_many(:builds) }
   it { is_expected.to validate_presence_of :sha }