Assign approvers based on code owners

The CE port is at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22513

What does this MR do?

Assign approvers based on code owners

• Only allow assigning when code owner feature is available
• Update owner when commit is pushed
• Give code owner as option when creating merge request
• Remove code owner as approver suggestion

Context

The major changes are:

1. overall_approvers method now returns User instead of Approver. This is because it needs to include code owners, which are dynamically calculated therefore does not have a corresponding Approver column.
2. Due to the above, calls to overall_approvers are changed to work with User instead.
3. And due to the above, one action is added to enable deleting of approver by using user_id instead. This is because we only have User instead of Approver, and if we want to get an user's approver it would cause extra database queries. At the end I decided to allow deleting of approvers by using user_id instead.
4. For code push, we need to compute the difference in code owners, before and after the push.

What are the relevant issue numbers?

Closes #1012 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

app/models/merge_request.rb

@@ -398,6 +398,31 @@ def diff_size
     merge_request_diff&.real_size || diffs.real_size
   end
 
+  def touched_file_paths(merge_request_diff: nil)
+    diffs = if merge_request_diff
+              merge_request_diff
+            elsif compare
+              compare
+            else
+              self.merge_request_diff
+            end
+
+    if diffs.is_a?(MergeRequestDiff)
+      # Skip object allocation by fetching database columns directly
+      paths = diffs.merge_request_diff_files.pluck(:new_path, :old_path).flatten
+    else
+      paths = []
+      diffs.diffs.diff_files.each do |diff|
+        paths << diff.old_path
+        paths << diff.new_path
+      end
+    end
+
+    paths.compact!
+    paths.uniq!
+    paths
+  end
+
   def diff_base_commit
     if persisted?
       merge_request_diff.base_commit