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?

doc/user/project/merge_requests/merge_request_approvals.md

@@ -40,7 +40,7 @@ suitable to your workflow:
 The following can approve merge requests:
 
 - Users being added as approvers at project or merge request level.
-- Code owners related to the merge request ([introduced](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7933) in [GitLab Starter](https://about.gitlab.com/pricing/) 11.5).
+- [Code owners](../code_owners.md) related to the merge request ([introduced](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7933) in [GitLab Starter](https://about.gitlab.com/pricing/) 11.5).
 
 An individual user can be added as an approver for a project if they are a member of:
 
@@ -58,7 +58,7 @@ an eligible approver, unless [self-approval] is explicitly enabled on the projec
 ### Implicit approvers
 
 If the number of required approvals is greater than the number of approvers,
-then other users will become implicit approvers to fill the gap.
+other users will become implicit approvers to fill the gap.
 Those implicit approvers include members of the given project with Developer role or higher.
 
 ## Adding or removing an approval