Add approve quick action

When a MR can be merged, an approval might be missing which makes the
`/merge` quick action a bit weird as it doesn't execute. Through this
commit a `/approve` is added so the following will work:

```
/approve
/merge
```

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/1873

doc/user/project/quick_actions.md

@@ -51,6 +51,7 @@ discussions, and descriptions:
 | `/move path/to/project`    | Move this issue to another project | ✓ |               |
 | `/target_branch <Local branch Name>` | Set target branch    |       | ✓             |
 | `/wip`                     | Toggle the Work In Progress status |   | ✓             |
+| `/approve`                 | Approve the merge request      |       | ✓             |
 | `/merge`                   | Merge (when pipeline succeeds) |       | ✓             |
 
 

ee/app/services/ee/quick_actions/interpret_service.rb

@@ -72,6 +72,17 @@ module EE
         @updates[:epic] = nil
       end
 
+      desc 'Approve a merge request'
+      explanation 'Approve the current merge request'
+      condition do
+        issuable.is_a?(MergeRequest) && issuable.persisted? && issuable.can_approve?(current_user)
+      end
+      command :approve do
+        if issuable.can_approve?(current_user)
+          ::MergeRequests::ApprovalService.new(issuable.project, current_user).execute(issuable)
+        end
+      end
+
       def extract_epic(params)
         return nil if params.nil?
 

ee/changelogs/unreleased/zj-approval-slash-command.yml

+---
+title: Add approve quick action
+merge_request: 7989
+author:
+type: added

ee/spec/services/quick_actions/interpret_service_spec.rb

@@ -177,6 +177,30 @@ describe QuickActions::InterpretService do
         end
       end
     end
+
+    context 'approve command' do
+      let(:merge_request) { create(:merge_request, source_project: project) }
+      let(:content) { '/approve' }
+
+      it 'approves the current merge request' do
+        service.execute(content, merge_request)
+
+        expect(merge_request.approved_by_users).to eq([current_user])
+      end
+
+      context "when the user can't approve" do
+        before do
+          project.team.truncate
+          project.add_guest(current_user)
+        end
+
+        it 'does not approve the MR' do
+          service.execute(content, merge_request)
+
+          expect(merge_request.approved_by_users).to be_empty
+        end
+      end
+    end
   end
 
   describe '#explain' do