Added "developers can merge" setting to protected branches

CHANGELOG

app/assets/javascripts/protected_branches.js.coffee

app/controllers/projects/protected_branches_controller.rb

app/models/merge_request.rb

app/models/project.rb

app/services/git_push_service.rb

...iews/projects/protected_branches/_branches_list.html.haml

...s/projects/protected_branches/_protected_branch.html.haml

app/views/projects/protected_branches/index.html.haml

...9203051_add_developers_can_merge_to_protected_branches.rb

db/schema.rb

lib/gitlab/access.rb

lib/gitlab/git_access.rb

spec/lib/gitlab/git_access_spec.rb

spec/services/git_push_service_spec.rb