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>

modules/context/base.go

@@ -136,6 +136,10 @@ func (b *Base) JSONRedirect(redirect string) {
 	b.JSON(http.StatusOK, map[string]any{"redirect": redirect})
 }
 
+func (b *Base) JSONError(msg string) {
+	b.JSON(http.StatusBadRequest, map[string]any{"errorMessage": msg})
+}
+
 // RemoteAddr returns the client machine ip address
 func (b *Base) RemoteAddr() string {
 	return b.Req.RemoteAddr