Delete branch service with permission checks

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/services/delete_branch_service.rb

+class DeleteBranchService
+  def execute(project, branch_name, current_user)
+    repository = project.repository
+    branch = repository.find_branch(branch_name)
+
+    # No such branch
+    unless branch
+      return error('No such branch')
+    end
+
+    # Dont allow remove of protected branch
+    if project.protected_branch?(branch_name)
+      return error('Protected branch cant be removed')
+    end
+
+    # Dont allow user to remove branch if he is not allowed to push
+    unless current_user.can?(:push_code, project)
+      return error('You dont have push access to repo')
+    end
+
+    if repository.rm_branch(branch_name)
+      Event.create_ref_event(project, current_user, branch, 'rm')
+      success('Branch was removed')
+    else
+      return error('Failed to remove branch')
+    end
+  end
+
+  def error(message)
+    {
+      message: message,
+      state: :error
+    }
+  end
+
+  def success(message)
+    {
+      message: message,
+      state: :success
+    }
+  end
+end