Remove jQuery from username change prompt and fix its detection (#29197)

Remove jQuery from username change prompt and fix its detection (#29197)

- Switched to plain JavaScript
- Tested the user rename prompt toggling functionality and it works as
before
- Fixed bug that allowed pasting with the mouse to avoid the prompt

# Before

![before](https://github.com/go-gitea/gitea/assets/20454870/aa300ad7-612b-461e-bbb2-3f74b3b83ede)

# After

![after](https://github.com/go-gitea/gitea/assets/20454870/f2b5a51b-7b39-43c7-8a4a-62f1f77acae4)

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Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: silverwind <me@silverwind.io>