Add `/assign me` alias support for assigning issuables to oneself

Currently, when a user wants to assign an issue/MR to himself, he needs
to type his full username or select it from the suggested ones in the
dropdown list.
This commits suggest a faster solution which is typing `/assign me`

Closes #35304.

app/services/quick_actions/interpret_service.rb

@@ -511,7 +511,12 @@ module QuickActions
       users = extract_references(params, :user)
 
       if users.empty?
-        users = User.where(username: params.split(' ').map(&:strip))
+        users =
+          if params == 'me'
+            [current_user]
+          else
+            User.where(username: params.split(' ').map(&:strip))
+          end
       end
 
       users

spec/services/quick_actions/interpret_service_spec.rb

@@ -424,6 +424,26 @@ describe QuickActions::InterpretService do
       end
     end
 
+    context 'assign command with me alias' do
+      let(:content) { "/assign me" }
+
+      context 'Issue' do
+        it 'fetches assignee and populates assignee_ids if content contains /assign' do
+          _, updates = service.execute(content, issue)
+
+          expect(updates).to eq(assignee_ids: [developer.id])
+        end
+      end
+
+      context 'Merge Request' do
+        it 'fetches assignee and populates assignee_ids if content contains /assign' do
+          _, updates = service.execute(content, merge_request)
+
+          expect(updates).to eq(assignee_ids: [developer.id])
+        end
+      end
+    end
+
     it_behaves_like 'empty command' do
       let(:content) { '/assign @abcd1234' }
       let(:issuable) { issue }

spec/support/features/issuable_slash_commands_shared_examples.rb

@@ -279,6 +279,17 @@ shared_examples 'issuable record that supports quick actions in its description
         expect(issuable.subscribed?(master, project)).to be_falsy
       end
     end
+
+    context "with a note assigning the #{issuable_type} to the current user" do
+      it "assigns the #{issuable_type} to the current user" do
+        write_note("/assign me")
+
+        expect(page).not_to have_content '/assign me'
+        expect(page).to have_content 'Commands applied'
+
+        expect(issuable.reload.assignees).to eq [master]
+      end
+    end
   end
 
   describe "preview of note on #{issuable_type}" do