Use "is-loading" to avoid duplicate form submit for code comment (#28143) (#28147)

Use "is-loading" to avoid duplicate form submit for code comment (#28143) (#28147)

Backport #28143 by @wxiaoguang

Compare by ignoring spaces:
https://github.com/go-gitea/gitea/pull/28143/files?diff=split&w=1

When the form is going to be submitted, add the "is-loading" class to
show an indicator and avoid user UI events.

When the request finishes (success / error), remove the "is-loading"
class to make user can interact the UI.

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>