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_diff.rb

@@ -52,8 +52,24 @@ def self.find_by_diff_refs(diff_refs)
     find_by(start_commit_sha: diff_refs.start_sha, head_commit_sha: diff_refs.head_sha, base_commit_sha: diff_refs.base_sha)
   end
 
-  def self.second_to_last
-    order(id: :desc).offset(1).first
+  # @return [Hash] mapping from merge_request_id to second to last diff
+  def self.second_to_last(merge_requests)
+    sub_query = MergeRequestDiff.where(merge_request_id: merge_requests).select(
+      <<-SQL
+        (
+          SELECT id FROM merge_request_diffs AS temp
+          WHERE temp.merge_request_id = merge_request_diffs.merge_request_id
+          ORDER BY temp.id DESC
+          LIMIT 1 OFFSET 1
+        )
+      SQL
+    ).group(:merge_request_id)
+
+    diffs = MergeRequestDiff.where(id: sub_query)
+
+    diffs.each_with_object({}) do |diff, hash|
+      hash[diff.merge_request_id] = diff
+    end
   end
 
   def viewable?