Use fetch to send requests to create issues/comments (#25258)

Follow #23290

Network error won't make content lost. And this is a much better
approach than "loading-button".

The UI is not perfect and there are still some TODOs, they can be done
in following PRs, not a must in this PR's scope.

<details>


![image](https://github.com/go-gitea/gitea/assets/2114189/c94ba958-aa46-4747-8ddf-6584deeed25c)

</details>

routers/init.go

@@ -183,6 +183,8 @@ func NormalRoutes(ctx context.Context) *web.Route {
 	r.Mount("/api/v1", apiv1.Routes(ctx))
 	r.Mount("/api/internal", private.Routes())
 
+	r.Post("/-/fetch-redirect", common.FetchRedirectDelegate)
+
 	if setting.Packages.Enabled {
 		// This implements package support for most package managers
 		r.Mount("/api/packages", packages_router.CommonRoutes(ctx))