Replace Fomantic `loader` CSS module with our existing `is-loading` spinner. Only three places in the UI used this module, which are pictured here: imagediff: <img width="1237" alt="Screenshot 2023-08-22 at 22 18 01" src="https://github.com/go-gitea/gitea/assets/115237/b0d82531-f05e-43c6-9e5b-1bfc268c056d"> webauthn: <img width="894" alt="Screenshot 2023-08-22 at 22 05 05" src="https://github.com/go-gitea/gitea/assets/115237/7b583425-d944-474a-a57a-22a65bbd8b29"> heatmap (I removed the previous loading text, it was unreadable because it was tiny and on fast machines only visible for a fraction of a second): <img width="764" alt="Screenshot 2023-08-22 at 22 18 44" src="https://github.com/go-gitea/gitea/assets/115237/1c7472d6-3e17-4224-a992-d8c0b380cc73"> Also, heatmap container does not resize any more after loading now and previous duplicate id `user-heatmap` is gone. --------- Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
