Add ProjectLabel model

app/finders/issuable_finder.rb

app/helpers/labels_helper.rb

app/models/label.rb

app/models/project.rb

app/models/project_label.rb

app/policies/project_label_policy.rb

app/views/projects/labels/_form.html.haml

app/views/projects/labels/edit.html.haml

app/views/projects/labels/new.html.haml

...igrate/20160920191518_set_project_label_type_on_labels.rb

lib/banzai/filter/label_reference_filter.rb

lib/gitlab/google_code_import/importer.rb

spec/factories/labels.rb

spec/models/label_spec.rb

spec/models/project_label_spec.rb

spec/models/project_spec.rb