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>

tests/integration/attachment_test.go

@@ -83,7 +83,7 @@ func TestCreateIssueAttachment(t *testing.T) {
 	}
 
 	req = NewRequestWithValues(t, "POST", link, postData)
-	resp = session.MakeRequest(t, req, http.StatusSeeOther)
+	resp = session.MakeRequest(t, req, http.StatusOK)
 	test.RedirectURL(resp) // check that redirect URL exists
 
 	// Validate that attachment is available