Correct swagger annotations for enums, status codes, and notification state (#37030)

## ⚠️ BREAKING ⚠️

- delete reaction endpoints is changed to return 204 No Content rather
than 200 with no content.

## Summary

Add swagger:enum annotations and migrate all enum comments from the
deprecated comma-separated format to JSON arrays. Introduce
NotifySubjectStateType with open/closed/merged values. Fix delete
reaction endpoints to return 204 instead of 200.

services/forms/repo_form.go

@@ -521,7 +521,7 @@ func (f *InitializeLabelsForm) Validate(req *http.Request, errs binding.Errors)
 // swagger:model MergePullRequestOption
 type MergePullRequestForm struct {
 	// required: true
-	// enum: merge,rebase,rebase-merge,squash,fast-forward-only,manually-merged
+	// enum: ["merge","rebase","rebase-merge","squash","fast-forward-only","manually-merged"]
 	Do                     string `binding:"Required;In(merge,rebase,rebase-merge,squash,fast-forward-only,manually-merged)"`
 	MergeTitleField        string
 	MergeMessageField      string